The Evolution of Viruses - Zhdanov V. M. 1990

Special Section
Poxviruses

The poxvirus family (Poxviridae) is divided into two subfamilies: Chordopoxvirinae (vertebrate poxviruses) and Entomopoxvirinae (insect poxviruses). The first subfamily includes 6 genera: Orthopoxvirus, Parapoxvirus, Avipoxvirus, Capripoxvirus, Leporipoxvirus, and Suipoxvirus. Each genus comprises either a single species or, more commonly, multiple virus species. The second subfamily comprises 3 genera, classified based on the morphological features of virions, genome molecular weight, and host range: Coleopteran poxviruses (Colioptera), Lepidopteran poxviruses (Lepidoptera), and Dipteran poxviruses (Diptera). Unlike vertebrate poxviruses, insect-infecting Viruses are serologically unrelated both between genera and within the same genus; therefore, their Classification should be considered provisional. In addition, several poxviruses remain unclassified, including those infecting carnivores, elephants, raccoons, Yaba and Tana monkey viruses, as well as the Molluscum Contagiosum virus [Matthews R., 1982].

The Morphology of virions in both subfamilies is so characteristic that there was no need to subdivide representatives infecting such phylogenetically distant hosts into higher taxonomic ranks. Poxvirus virions are oval or brick-shaped with an axis ratio of 1.2–1.7. Their dimensions are 140–170×220–450 nm, approaching the size of the smallest Bacteria (such as rickettsiae, chlamydiae, and Mycoplasmas). Their Structure does not fit the Symmetry types of other viruses and features specific variations among representatives of different genera. Common to all viruses in this family is a biconcave core, an outer membrane, and lateral bodies that appear to compress the core (nucleoid).

The nucleoid contains DNA associated with Proteins. The exact packing mechanism remains unclear, although three proteins with molecular weights of 33,000, 28,000, and 12,000 have been isolated from the DNA-protein complex, with the latter being rich in Arginine. This suggests an analogy to the packaging of eukaryotic chromosomal DNA by histone and non-histone proteins. The nucleoid is surrounded by inner membranes, while the outer envelopes consist of Lipids and CARBOHYDRATES. These envelopes have a complex layered structure composed of tubular or globular units. Different viral genera vary primarily in the fine structural details of their outer envelopes.

The viral genome is a linear double-stranded DNA molecule with a Molecular Weight of 85×10⁶–250×10⁶, whose ends are covalently linked, forming terminal loops and inverted tandem repeats. Genome molecular weight cannot serve as a reliable taxonomic criterion, as it ranges from 85×10⁶ (parapoxvirus) to 200×10⁶ (avipoxvirus) even among serologically related viruses. The G+C content in vertebrate poxviruses accounts for 5–7.5% of the virion mass, with a Nucleotide Composition of A: 29.5%, C: 20%, G: 20.6%, and T: 29.9%. The vaccinia virus genome contains approximately 240,000 Base Pairs and is 82 µm long. The unusual Stability of the DNA of this and other related viruses is due to covalent bonds between complementary strands at both molecular ends, located roughly 50 NUCLEOTIDES from the termini [see Slepushkin A. N., 1982]. As demonstrated in studies of vaccinia virus, its DNA—like that of many other viruses—contains terminal inverted repeats that are hypothesized to play a crucial role in METABOLISM/36.html">DNA Replication. In vaccinia virus, these repeats are quite large, with a molecular weight of about 7×10⁶ (roughly 10,000 base pairs). They can be partially transcribed, encoding the synthesis of early mRNAs [Wittek R. et al., 1980].

The poxvirus genome encodes several hundred proteins (Glycoproteins and Phosphoproteins) with molecular weights ranging from 80,000 to 200,000. According to various authors, the number of virus-specific proteins varies among different viruses. It was previously noted that vaccinia virions contain more than 80 proteins, accounting for about 60% of the total genome mass [Fenner F., 1979]. Yaba monkey poxvirus was found to contain 37 structural proteins with molecular weights between 10,000 and 220,000 [Fenger T., Rouhandeh H., 1976]. More recent data indicate that vaccinia virus possesses 279 proteins, including 13 glycoproteins [Carrasco L., Bravo R., 1985]. To date, only a few virion and non-virion proteins have been identified. Virus neutralization is associated with a surface protein (58,000), while hemagglutination is linked to a glycoprotein (85,000). Other major proteins are designated by the symbols 2b, 4a, 4b, 6a, 6b, 8, and 11b; two proteins (4a and 4b) comprise 50% of the total core proteins, with the core itself accounting for about 50% of the virion mass. The outer envelopes contain 8 proteins, and the surface proper contains 5 proteins. The virions contain, or generate during reproduction, at least 10 Enzymes, including RNA polymerase (virion transcriptase), enzymes that catalyze the formation and methylation of cap structures essential for mRNA synthesis and Processing, as well as protein kinase and thymidine kinase. Viral DNA Synthesis is also likely catalyzed primarily by virus-specific enzymes [Slabaugh M., Matthews C., 1984].

Out of approximately 10 major virion Antigens, one cross-reacts with all members of vertebrate poxviruses. Serological relationships are even more pronounced among viruses within the same genus. During the Cytology/cytology/16.html">Early stages of vaccinia virus replication, a 19,000-molecular-weight protein is synthesized that resembles mammalian embryonic growth factor (the potential significance and origin of this protein are discussed below). Due to the complex processes of viral Protein Synthesis AND maturation, as well as virion morphogenesis, the antigenic profiles of intracellular and extracellular viruses differ.

Poxvirus replication (Fig. 61) occurs step-by-step in the Cytoplasm, including primary uncoating (uncoating I), synthesis of early mRNAs and proteins, secondary uncoating (uncoating II), DNA replication, synthesis of late mRNAs and virion structural proteins, their glycosylation and phosphorylation, formation of virion precursors, maturation, and release from The Cell [Shida H., 1986].

Although there is Evidence indicating the involvement of the Cell Nucleus in poxvirus reproduction [Miningan H. et al., 1985], it is not strictly required, as the entire replication cycle can take place in enucleated Cells [Villareal E. et al., 1984]. At the same time, virus reproduction is closely linked to cellular metabolism, even when cell-dependent Protein synthesis is completely excluded. This is supported by the successful replication of vaccinia virus strains deficient in certain genes (such as the thymidine kinase Gene), the possibility of substituting this gene with a herpesvirus gene, and the presence of sizeable non-replicating regions within the viral genome. The utilization of these non-coding regions forms The basis of Introduction/32.html">Genetic Engineering research, in which vaccinia virus with inserted foreign genes serves as an expression vector (see below).

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Fig. 61. Schematic diagram of the vaccinia virus replication cycle.

Following the adsorption of virions to cell membranes and their entry into the cell via endocytic vacuoles, fusion of the cellular and viral membranes occurs, followed by primary virion deproteinization and the release of cores (nucleoids), which become metabolically active after this initial uncoating stage. The synthesis of early mRNAs, cap formation, and polyadenylation are mediated by virion-associated enzymes without RNA splicing [Venkatesan S., Moss B., 1981]. The regulatory regions of poxvirus genes contain adenine- and thymine-rich (TATAAT-like) sequences but otherwise differ from prokaryotic and eukaryotic genes. During early stages, 14% of The Genome is transcribed, yielding early proteins such as thymidine kinase, endonucleases, other viral enzymes, and a related protein. Thus, virion deproteinization comprises two stages: a cell-dependent phase that removes the outer envelopes, and a virus-dependent phase that achieves deeper deproteinization of the DNA to allow for its replication.

The sites of DNA Replication and virion assembly are specialized cytoplasmic structures modified by the virus, commonly referred to as viral factories. More than 100 proteins synthesized During the first hours of infection drive the second uncoating stage, Viral DNA Replication, and late gene Transcription. Key Enzymes synthesized during this stage include thymidine kinase, DNA polymerase, polynucleotide ligase, and other DNA-synthetic enzymes.

Viral DNA replication involves the Cleavage of terminal loops and self-priming at the 3'-ends of the viral DNA. However, many details of viral DNA synthesis remain poorly understood, leading to the proposal of several alternative Replication Models. Late transcription encompasses the majority of the genome, during which structural proteins destined for the virion are translated. It is estimated that 1/3 of the poxvirus genome encodes early proteins, 1/3 encodes structural proteins, and 1/3 encodes late non-structural proteins [Pennington T., 1976]. Late gene transcription appears to involve cellular RNA polymerase II. Some of the proteins synthesized at this stage undergo proteolytic cleavage, which is coupled with the assembly of virions and their precursors.

Virion assembly takes place within the viral factories and is a multi-step process. First, progeny DNA associates with internal proteins to form dense filaments and granules. Next, an envelope forms as a smooth membrane enclosing the electron-dense material, yielding immature virions (provirions) that appear as vesicular structures where nucleoids (cores) and lateral bodies subsequently develop. Condensation of the internal material and membranes, along with The formation of surface structures, completes the maturation of infectious virions. Virions are transported out of the cell via intracellular pathways, and some are released following cell lysis.

Poxviruses induce a profound suppression of host macromolecular synthesis, with host pathways blocked by both virion proteins and proteins synthesized de novo during viral reproduction. Certain poxvirus family members stimulate infected cell proliferation and neoplastic transformation (fibroma virus, Yaba virus, and molluscum contagiosum virus). Poxviruses are characterized by genetic phenomena such as intra-genus recombination, marker rescue [Nakano E. et al., 1982], and non-genetic reactivation between different genera of vertebrate poxviruses. Naturally circulating poxviruses undergo active recombination; a notable example is the malignant rabbit virus, a recombinant comprising 10% of the fibroma virus genome and 90% of the myxoma virus genome [Block W. et al., 1985].

A more detailed description of poxvirus genetics can be found in specialized monographs and reviews [see Gendon Yu. Z., 1975]. Here, we wish to highlight several important features that are characteristic not only of poxviruses but also of other previously mentioned DNA viruses (Herpesviruses, Baculoviruses, Adenoviruses, etc.). First, the DNAs of closely related poxviruses or mutants readily recombine, generating both homologous and heterologous recombinants. In the latter case, the resulting heteroduplexes may incorporate foreign genes. Second, recombination can occur not only between complete genomes of two poxviruses, but also between a full genome and a relatively small genomic fragment (4×10⁶) introduced via a plasmid during cotransfection. Third, the poxvirus genome contains large regions conventionally designated as non-essential for replication. These span approximately 25,000 base pairs, or more than 10% of the entire genome [Smith G., Moss B., 1983]. These regions likely encode replication-related proteins and other factors that can be substituted by cellular proteins (such as thymidine kinase), or perhaps factors determining viral virulence. All these properties make vaccinia virus a highly versatile vector for expressing foreign genes. Rather than citing the extensive literature on The Use of vaccinia virus in genetic engineering, we refer the reader to the WHO expert memorandum on this subject [WHO, 1985]. Crucially, genetic engineering operations rely on harnessing natural genetic interactions occurring within this and other virus groups.

Incidentally, this very property of poxviruses hindered the proposed use of a recombinant vaccinia virus expressing the rabies virus glycoprotein gene for fox immunization. When foxes consume bait laced with a suspension of such virus, they and other carnivores develop vaccinia stomatitis accompanied by simultaneous Immunity to both vaccinia and rabies. However, the WHO Expert Committee strongly objected to this field trial planned in Argentina, citing the potential risk of generating naturally occurring poxvirus recombinants with enhanced neurovirulence conferred or linked to the rabies virus gene inserted into the vaccinia genome.

Poxviruses cause a wide spectrum of diseases, predominantly manifested as rashes on the Skin and mucous membranes. Accordingly, in animals, infections are transmitted through contact, contaminated feed, and Water, whereas in humans, transmission also occurs via airborne droplets. A distinct category includes neoplastic conditions affecting either the outer integument or deeper Tissues (e.g., fibromas), in which case Blood-sucking insects may play a role in transmission.

The enormous size of the poxvirus genome—comparable to that of the smallest bacteria (mycoplasmas, rickettsiae, and especially chlamydiae)—along with the autonomy of replication and transcription driven by virus-specific enzymes and the absence of RNA splicing, raises two fundamental questions. Is it legitimate to consider poxviruses true viruses? Or are they products of degenerative, adaptive bacterial evolution, perhaps even closer to the cellular line than Organelles such as Plastids and Mitochondria? Indeed, chlamydiae—the smallest bacteria—have evolutionarily degenerated to the point of losing their own energy-generating systems, effectively becoming metabolic parasites. Their biological properties differ from those of poxviruses "only" by the presence of ribosomal protein-synthesizing machinery. However, this single distinction marks the dividing line between viruses and cellular life forms, leaving no compelling grounds at present to exclude poxviruses from the viral realm.

Unfortunately, evolutionary relationships between poxviruses and prokaryotes (eukaryotes and/or archaebacteria) have not yet been rigorously tested, as such analyses, including Phylogenetic Tree construction, are typically based on ribosomal RNA sequences (23S, 16S, 5S). These are present even in descendants of anciently diverged endosymbionts (Mitochondria and Plastids) but are entirely absent in poxviruses.

Moreover, despite the substantial autonomy of the poxvirus genome from the host cell genome—including viral DNA replication—evidence exists for genetic cross-talk between poxviruses and their eukaryotic hosts. For instance, Analysis of the 19,000-molecular-weight protein synthesized during early vaccinia virus replication revealed an unusual arrangement of Cysteine and Glycine residues closely matching that found in two cellular proteins: transforming growth factor (TGF) and epidermal growth factor (EGF). These two functionally similar proteins of comparable size (50 and 53 amino acid residues, respectively) stimulate cell proliferation. The former is found in many tumors and embryonic tissues, while the latter occurs in the submandibular glands and urine of humans. It has been hypothesized that all three proteins, along with certain other animal proteins [Blomquist M. et al., 1984], share a common evolutionary origin, and a phylogenetic tree has even been constructed for them (Fig. 62). While lacking immunological cross-reactivity, these three proteins share functional activity. The vaccinia virus growth factor-like protein is an early secreted (non-structural) protein whose exact function remains to be elucidated.

Fig. 62. Phylogenetic tree of the vaccinia virus protein and homologous mammalian domains. Dashed ovals indicate unresolved Branches of the tree.

Poxvirus genomes share a key feature with Eukaryotic Cell genomes: the presence of tandem repeats within their inverted terminal repeats, as demonstrated in studies of vaccinia virus. These repeats are believed to facilitate the circularization of single-stranded DNA during replication. Ranging from 70 to 150 base pairs in length, they are repeated in tandem 13 to 30 times [Wittek R., Moss B., 1980].

Thus, the precise origin of poxviruses remains elusive. Their massive genome, cell-independent replication, and lack of splicing ally them with bacteria. Yet their genome structure also mirrors that of eukaryotes, and several of their genes are clearly of eukaryotic origin—unless they represent the end-product of advanced molecular convergence.

Despite the strong structural and morphological similarities between the virions of vertebrate and insect poxviruses, the presence of at least 4 enzymes with identical Functions, replication in the cytoplasm, and several other properties indicating a common origin for both groups, much remains unclear about the Evolution of the poxvirus family as a whole. Insect poxviruses likely have a more ancient origin simply because insects are far more primitive life forms than warm-blooded vertebrates. Poxviruses have not been found in marine invertebrates; assuming this is an accurate reflection of current knowledge rather than a gap in it, one can infer that poxviruses emerged no earlier than the Silurian period, i.e., approximately 400 million years ago, or even later during the Carboniferous.

Their ADAPTATION TO A new ecological niche can be associated with the evolution of blood-sucking insects, given that some vertebrate poxviruses can still be transmitted by vectors of this type. Interestingly, all currently known vertebrate poxviruses infect warm-blooded animals (mammals or birds), whereas insect poxviruses infect both non-hematophagous insects (Lepidoptera) and hematophagous ones (Diptera), including blood-sucking mosquitoes that feed on warm-blooded species. Therefore, The Emergence of vertebrate poxviruses should be assigned to a comparatively late period—the Jurassic or Cretaceous, i.e., 120–150 million years ago or later. This precisely explains the moderate divergence observed among the 6 groups (genera) of vertebrate poxviruses, which extends to the preservation of shared antigens and even broader antigenic relationships among viruses within the same genus.

The evolution of these viruses proceeded through the occupation of novel ecological niches, followed by narrow specialization to the point where a given virus infects only a single host species. This is how the poxviruses of buffaloes, camels, rabbits, mice (vaccinia virus group), milker's nodules, contagious ecthyma, bovine pustular stomatitis (parapoxvirus group), canaries, pigeons, quails, swallows, sparrows, turkeys (avian poxvirus group), sheep, goats, rabbit fibroma and myxoma, and pigs originated and evolved. Animal domestication, on the one hand, intensified viral specialization (sheep, goat, and camel pox), and on the other hand, accelerated viral dissemination, resulting in the same domestic animal species becoming susceptible to multiple poxviruses. For instance, cattle are susceptible to vaccinia, cowpox, parapoxvirus, and milker's nodules viruses, while rabbits are affected by rabbit pox, fibroma, and myxoma viruses. However, the question of strict host Specificity is not entirely straightforward. We often speak of cowpox, but in reality, this refers to a disease contracted by cows whose primary reservoir is likely rodents. Monkeypox virus has been isolated from squirrels [Khodakevich L., 1985]. An analysis of cowpox virus revealed a broad pathogenic spectrum spanning 9 mammalian orders (cattle, kangaroos, rhinoceroses, dolphins, felines, rodents). All these data suggest that rodents serve as the primary reservoir of the pathogen in nature and the source of infection for cattle and other mammals [Berkovitz E., Pogo B., 1984]. According to WHO data, cowpox is essentially a disease of cattle, whereas monkeypox is a disease of monkeys. In the latter case, squirrels have been identified as the reservoir for monkeypox, with monkeys acting as secondary sources of infection [WHO, 1986].

Molecular biological studies of the Shope fibroma virus demonstrated that it is most closely related to orthopoxviruses while simultaneously harboring a genomic region akin to that of leporipoxviruses [Berkovitz E., Pogo B., 1985]. Thus, poxvirus recombination occurs in nature, and certain viruses unquestionably have a recombinant origin [Berkovitz E., Pogo B., 1985].

Before concluding this section, we would like to recall the evolution of one particular poxvirus—the rabbit myxomatosis virus. This evolutionary process unfolded before our eyes [Fenner F., 1979]. This highly lethal virus was introduced in Australia to control overpopulated rabbit populations imported from Europe that were causing severe agricultural damage. The interaction between the two populations—the virus and the host—led to The Development of a persistent infection in the animals instead of an acute, lethal one. This clearly demonstrated the natural Selection of genetically resistant animals and a persistent virus, which altered the original character of the infection to varying degrees.

However, let us return to the initial and subsequent Stages of the evolution of animal poxviruses [Zhdanov V. M., Lvov D. K., 1984].

In poxvirus infections of birds and mammals, the skin and mucous membranes are primarily affected. The disease is accompanied by viremia and prolonged virus carriage. Mouse ectromelia is characteristic in this regard, as carriage can be lifelong. All these diseases are transmitted either through contact, alimentary routes, or via blood-sucking insects. The infection typically strikes young animals, in whom it can take an acute course. Survivors become long-term—and often, as in the case of ectromelia, lifelong—carriers, subsequently transmitting the virus to their offspring.

Infections of this type could have emerged long ago, as the prolonged persistence of the pathogen and, consequently, the extended infectious period ensured the Circulation of the virus under conditions of low-grade epizootics. The fact that each poxvirus currently infects a single species or a few closely related species points to a long-term co-evolution of parasites and their hosts. Poxvirus evolution in this direction continued even after animal domestication; because these viruses descended from multiple ancestors, a situation arose where certain domesticated animal species became susceptible to several different poxviruses. As already mentioned, cattle experience cowpox, lumpy skin disease, papular stomatitis, and DISEASES ASSOCIATED WITH parapoxviruses (contagious ecthyma, milker's nodules, etc.). All these conditions are caused by distinct viruses classified into different genera. Furthermore, there is another disease—buffalo pox—caused by a specialized virus. Sheep suffer from sheep pox and pustular dermatitis, which are caused by different poxviruses. This also applies to certain wild animals: rabbits contract rabbit pox, fibroma, and myxoma. Some of these diseases have limited geographic ranges, but overall, poxvirus infections in domestic animals are ubiquitous.

A comparison of poxvirus infections in wild and domestic animals reveals both Similarities and differences. The former are characterized by a prolonged infectious period, carrier states, and frequently a chronic clinical course. Among domestic animal infections, those transmitted by parapoxviruses and, likely, avian poxdiphtheria come closest to this type. In contrast, the pox of sheep, goats, cows, horses, pigs, and camels is characterized by a more acute course and the absence of long-term carrier states. Infections of this latter type could not have become established among wild populations; therefore, it can be concluded that poxvirus diseases in domestic livestock arose relatively late, following animal domestication by humans—that is, at a time when large herds of these animals already existed.

The domestication of animals and the emergence of large herds drove the evolution of parapoxvirus-like diseases toward acute clinical courses, which ensured more rapid transmission of pathogens within the herd. A more acute disease course stimulated the development of a more robust immunity, precluding the possibility of prolonged latent infection. Such an infection could not have persisted among wild animals, but within large herds of domestic livestock under conditions of close contact, the survival of an acute infection without a prolonged carrier state became viable. The presence of numerous animal species domesticated by humans created opportunities for the mutual exchange of parasites (viruses), the formation of ecological variants, and the emergence of independent diseases resulting from the adaptation of poxviruses to the Organism of domestic animals. It must be emphasized that various pox diseases likely arose among herd-forming animals, whereas adaptation to animal species not managed in herds was far less probable. It is hardly surprising, therefore, that known acute poxvirus infections do not affect domestic species such as cats and dogs. They are prevalent exclusively among herd animals, poultry (avian poxdiphtheria), and murid rodents (ectromelia).

Thus, even during early stages of societal development (the late barbarian period), when large herds of domestic livestock appeared, conditions were established for the emergence of acute and subacute poxvirus diseases in these animals. Transmission occurred predominantly via the alimentary route, which is why these viruses cause lesions not only on the skin but also on mucous membranes. Because transmission was alimentary, the viruses could survive only if they possessed high resistance to adverse environmental factors. Indeed, the causative agents of pox infections in wild animals are notable for their resilience and ability to persist in the environment for extended periods.

Pox diseases originated in the Old World, given that prior to European contact, the Americas possessed no domesticated animals other than llamas, and llamas do not contract natural pox infections. The earliest large herds of domestic livestock appeared in Southwest and Central Asia, as well as North Africa (Egypt). It was most likely here that poxvirus diseases of domestic animals first arose.

THE ORIGIN OF human smallpox is currently much better understood than it was 20 years ago. It would be natural to look for the precursors of the human smallpox virus among the Orthopoxvirus genus, which includes monkeypox and cowpox viruses that are pathogenic to humans. When examining this question previously [Zhdanov V. M., 1963], we favored the cowpox virus. However, in recent years, following the global eradication of smallpox, researchers' attention has shifted toward zoonotic human pox infections, particularly human monkeypox. By early 1984, more than 60 such cases with several fatalities had been registered in certain countries of Equatorial Africa. Laboratory Research Methods allow for the differentiation of monkeypox and smallpox viruses, as well as smallpox and vaccinia viruses—specifically, by detecting antigens specific to the three closest viruses in this group: MO (monkeypox virus antigen), VA (antigen common to human smallpox and vaccinia viruses), and VC (vaccinia virus antigen). Through the cloning of smallpox strains isolated from monkeys, S. S. Marennikova et al. (1972, 1976) isolated so-called white variants that closely resemble the human smallpox virus in their properties.

Similar variants have also been isolated from wild rodents in the same regions of Equatorial Africa (Table 21).

Based on the data in Table 21, one can hypothesize that the source of the human smallpox virus may have been white variants of monkeypox or rodent poxviruses from Equatorial Africa. It is known that monkeypox virus is pathogenic to humans, causing a generalized disease process, yet under normal conditions, it is not transmitted from human to human. However, one cannot rule out the possibility of contact transmission between humans; such a route of spreading the infectious agent is entirely plausible under primitive living conditions.

The generalization of the disease process sharply increases the contagiousness of the patient because the presence of a rash on the mucous membranes of the Oral Cavity and oropharynx (enanthema) and their maceration create new additional opportunities for airborne droplet transmission. Patient contagiousness rises dramatically, making the development of independent outbreaks among people possible. One must assume that outbreaks of this nature occurred repeatedly; however, under primitive communal conditions, they could not spread widely and remained confined within the limits of a family clan or tribe.

Table 21. Comparative properties of certain poxviruses [Fenner F., 1977]

Note: + yes, — no.

Increased population crowding and interhuman ties, along with the formation of large states, created conditions conducive to the broader dissemination of this infection type. Individual viral strains characterized by enhanced pathogenicity and a propensity to cause a generalized process could trigger major epidemics under these circumstances. It is conceivable that more than one such epidemic arose during this period; having affected a sizable number of individuals without achieving sustained transmission, the virus would die out, lacking the capacity for further spread within human society. Undoubtedly, the formation of human smallpox infection occurred not merely through the selection of minor viral Mutations driving increased pathogenicity; an escalation in virulence could also happen abruptly via mutation. One such leap could lead to a sharp surge in pathogenicity, endowing the virus with The ability to cause a generalized disease process. Within a large state characterized by high population density, this resulted in the pathogen breaking away from its ancestral animal reservoir and circulating exclusively among humans. A vast territorial expanse combined with a large—and in some instances high-density—population ensured the continuity of the epidemic process, which would have been impossible under primitive communal arrangements.

The further evolution of smallpox as an infection was governed, on the one hand, by societal development conditions and, on the other, by The Nature of human physiological responses. Viral strains that triggered an acute generalized process accompanied by an abundant rash possessed a higher survival advantage, as such diseases were accompanied by massive virus shedding and the infection of numerous surrounding individuals. Consequently, evolutionary selection favored strains causing an acute disease course. These strains provoked a more vigorous host response coupled with strong, protective immunity, which precluded the long-term persistence of the pathogen within the body and the establishment of a carrier state. Strains inducing excessively severe illness with high mortality rates likewise failed to become established, as population extinction led to the eradication of the virus itself. Thus, through the interplay of these conflicting selective pressures, an infection with an acute course, high infectivity, and robust post-infection immunity—which ultimately became variola vera—was shaped.

It remains to examine the timing and geography of smallpox's emergence. The distinct Clinical presentation of smallpox and its characteristic sequelae (scars) suggest that this disease could not have gone unnoticed, even in antiquity.

The earliest evidence of smallpox comes from the discovery of vesicular rashes on an Egyptian mummy of the 9th Dynasty (1200–1100 BCE), which are recognized as manifestations of smallpox. Indications pointing to the existence of smallpox in Egypt date back even further—to the Papyrus of Amenhotep I, dating from 3730–3710 BCE [Bashenin V., 1938]. This infection is likely of equal antiquity in Southeast Asia, where primitive vaccination attempts against it were documented as early as the 12th century BCE [Vogralik G., 1935]. F. Burnet (1945) took a more cautious view of these data, suggesting that the first reliable records of smallpox date from the 6th–11th centuries CE.

The data presented allow for the Conclusion that smallpox originated in the ancient states of North Africa, Southeast Asia, and the Eastern Mediterranean, from whence it spread to other countries. Smallpox emerged at the dawn of civilization and must therefore be considered one of the most ancient infections transmitted via the airborne droplet route.

In the evolutionary trajectory of smallpox, we encounter a phenomenon of profound biological significance. The Transformation of a zoonotic infection (monkeypox) into an anthroponosis (human smallpox)—that is, the detachment from an ancestral reservoir and adaptation to a new one—was accompanied by a fundamental shift in the transmission mechanism. An animal infection transmitted via contact and alimentary routes evolved into a human infection transmitted via airborne droplets. It was precisely this change in the transmission mechanism that made the entrenchment of smallpox among humans possible, albeit only at a specific stage of human social evolution. Having become an epidemiologically autonomous disease with two variants (variola major and alastrim), modern smallpox still bears the "imprint" of its origins: antigenic affinity with monkeypox, dermatotropism of the agent, and high environmental stability of the virus. These traits are otherwise uncharacteristic of infections spread via the airborne droplet route.

The subsequent history of smallpox has been traced in considerable detail. The first historically recorded smallpox epidemic occurred in the 11th century in the Middle East during the Arab-Abyssinian War. In the 6th century, outbreaks emerged in Italy and France, while in the early 7th century, smallpox was brought by the Arabs to Egypt and subsequently spread along the entire European coastline of the Mediterranean. By the mid-13th century, an epidemic was observed in Iceland, and the 16th century saw devastating outbreaks strike many European countries. From that time on, smallpox became endemic in Europe, affecting primarily the pediatric population. Following the Discovery of the Americas, the disease was introduced to the Caribbean islands and the mainland, causing catastrophic epidemics there. By the 16th century, smallpox was almost as much of an "inevitable" disease as measles is today. H. Vogralik (1935) noted that from the time smallpox appeared in Europe until the end of the 18th century, it claimed the lives of at least 150 million people. According to other estimates, in the 17th and 18th centuries, over 10 million people contracted smallpox annually in Europe, with about 1.5 million dying from it. In Russia, up to 0.5 million people died from smallpox annually during this period.

Following Edward Jenner's discovery of smallpox immunization, human history witnessed the first active human intervention in the epidemic process. However, this measure remained underutilized for a long time, and it was not until the 20th century that the epidemic spread of smallpox was halted in the most developed nations. Even then, the disease continued to be "imported" annually into Europe and North America, predominantly from the colonial countries of Southeast Asia and Africa, while serious attempts to limit its epidemic spread were virtually nonexistent. In the Soviet Union, smallpox was eradicated in 1936, which not only put an end to its epidemic propagation but also provided reliable protection for the country against importation from abroad. This is evidenced by the rarity of such importations in subsequent years, despite extensive borders with highly endemic countries and a significant expansion of economic and cultural ties worldwide. The last importation of smallpox into the USSR (Moscow) from India occurred in December 1959. Although the outbreak, unrecognized in time, affected 36 people, it was rapidly suppressed through quarantine measures and the immunization of Moscow's entire population.

A new phase in the fight against smallpox began in the post-war period, when many nations shook off colonial rule and embarked on independent development. Under these conditions, the global eradication of smallpox became feasible. At the proposal of the Soviet Union [Zhdanov V. M. et al., 1959], the WHO Assembly adopted a resolution in 1958 calling for the worldwide eradication of smallpox.

During its first decade, this effort lacked sufficient momentum, and by 1967, smallpox was still being reported in more than 40 countries. The situation changed when the WHO launched the "Intensified Smallpox Eradication Programme" (1967–1976). Active case detection and targeted ring vaccination around patients were implemented. Cash rewards were introduced for reporting smallpox cases. These measures proved highly effective: initially, the number of registered cases rose due to improved surveillance, but it subsequently plummeted. Variola major was defeated first, with the last case recorded on October 16, 1975, in Bangladesh. Two years later, on October 26, 1977, the final case of Variola minor was registered in Somalia. Thus, the global eradication of smallpox was successfully achieved [Ladny I. D., 1985]. Throughout this period, the Soviet Union provided substantial material and personnel support to the global eradication program. Numerous Soviet specialists worked in Asia and Africa until the final smallpox foci were eliminated, and the Soviet government donated over 1.5 billion doses of smallpox vaccine to the WHO and individual countries. Since 1977, smallpox has no longer threatened humanity, and the CAUSATIVE AGENT OF the disease is preserved in only a few specialized laboratories for ongoing research.