BIOLOGY Volume 2 - A Guide to General Biology - 2004

15. HEALTH AND DISEASE

15.3. Infectious Diseases

Infectious diseases are caused by organisms that colonize individuals of other species and live in or on them as parasites. The causative agents of such diseases are referred to as pathogens. Some Examples are listed in Tables 15.3–15.6. Cholera, tuberculosis, malaria, AIDS, typhoid fever, paratyphoid fever, and salmonellosis will be discussed in more detail in subsequent sections.

The defense mechanisms of The Human Body against pathogens were covered in Chapter 14 (see sections 14.8.5 and 14.9).

The following terms are commonly used when discussing infectious diseases.

Etiology — the set of causes responsible for a disease.

Epidemiology — The Study of factors contributing to the spread of a disease within a given population.

Causative agent — the Organism (pathogen) that causes a disease.

Vector — an organism that transmits a pathogen from an infected individual to a healthy one (of the same or a different species); for example, the mosquito is a vector (but not the causative agent) of malaria.

Class="center">Table 15.3. Some common viral diseases in humans (grouped according to their mode of transmission)

	Name	Causative Agent	Mode of Transmission	Symptoms	Vaccine Type
	Influenza	A myxovirus (DNA-containing virus); three types: A, B, and C with strains of varying virulence	Airborne droplet	Sudden onset of fever accompanied by headache, sore throat, and Muscle aches. Infection of the Upper Respiratory Tract epithelium, Trachea, and Bronchi. The illness lasts for a week, but residual effects may persist for a month. Secondary bacterial infection of the Lungs leading to Pneumonia cannot be ruled out	Inactivated virus (requires proper Selection of the causative strain)
	Common cold (ARI)	Various Viruses, most commonly rhinovirus (RNA-containing virus)	Airborne droplet	Irritation of the Nose and bronchi: runny nose, sneezing, and coughing. Usually restricted to the upper respiratory tract. Secondary bacterial infection is possible	Live or inactivated virus (intramuscular injection); low efficacy due to the many strains of rhinovirus involved
	Smallpox*	Variola virus (DNA-containing virus)	Airborne droplet or contact with Skin lesions, clothing, bedding, etc. of an infected person	High fever and generalized aches. Involvement of the respiratory tract. A rash appears after two days and spreads over the entire body. Secondary bacterial infection of the lesions leads to deep ulcers and skin scarring	Attenuated virus (applied to skin scratches); vaccination is no longer performed as there is no risk of infection
	Mumps (epidemic parotitis)	A paramyxovirus (RNA-containing virus)	Airborne droplet (or via infected saliva entering the Mouth)	Primarily affects children. Fever followed by inflammation of the parotid (salivary) glands (unilateral or bilateral), lasting about 10 days. The Testes, Ovaries, and Pancreas may also be involved. Orchitis in post-pubertal males carries a risk of sterility	Attenuated virus
	Measles	A paramyxovirus (RNA-containing virus)	Airborne droplet	Primarily affects children. Pharyngitis, runny nose, lacrimation, cough, and fever. White spots on the buccal mucosa inside the mouth (Koplik's spots). After two days, a reddish rash appears near the hairline on the neck and then spreads across the entire body. Recovery occurs within a week, but the virus may affect the Kidneys and Brain. Secondary bacterial infections are possible	Attenuated virus
	Rubella	Rubivirus (RNA-containing virus)	Airborne droplet	Primarily affects older children and adults. Involvement of the respiratory tract, cervical Lymph Nodes, eyes, and skin. Mild fever, skin rash disappearing within three days. Complications are rare; however, the risk of blindness, deafness, and other serious birth defects in children born to women infected During the first four months of Pregnancy reaches 20%	Attenuated virus: administered primarily to girls, as the most serious complications arise during pregnancy
	Poliomyelitis	Poliovirus (a picornavirus, classified as an RNA-containing virus); three strains are known	Airborne droplet or fecal-oral	Fever, headache, and neck stiffness, less commonly affecting other body parts. Motor Neurons supplying Muscles are destroyed, leading to paralysis and muscle atrophy (Artificial ventilation may be required). Most paralysis cases occur in children aged 4–12, though adults can also be affected	Attenuated virus administered orally, usually on a sugar cube
	Yellow fever	An arbovirus (RNA-containing virus) transmitted by Arthropods	Bites of vectors, specifically mosquitoes and ticks	Fever, headache, backache, nausea, and epigastric tenderness. Involvement of Blood vessel linings and the Liver. On the fourth day, vomiting of blood and Bile (resembling "coffee grounds"). Yellowing of the eyes. Black stools due to digested blood	Attenuated virus (vector control is also crucial)
	AIDS (Acquired Immunodeficiency Syndrome)	HIV (HUMAN IMMUNODEFICIENCY VIRUS) — a retrovirus (RNA-containing virus)	Contact with infected Body Fluids (especially blood), excluding saliva. Transmission can occur via sexual intercourse (homosexual and heterosexual)	Laboratory signs of infection, such as Antibodies to the virus, though symptoms are mild (swollen lymph nodes). Approximately 25% of HIV-positive individuals develop AIDS-related complex, which includes loss of appetite and weight, fever, persistent dry cough, white patches on mucous membranes and skin (candidiasis caused by the fungus Candida albicans), pneumonia, tuberculosis, and other conditions caused by immune suppression	None
	Hepatitis B	A DNA-containing virus	Contact with infected blood. Transmission can occur via sexual intercourse	Incubation period ranges from 6 weeks to 6 months. The liver is affected. Symptoms resemble influenza, accompanied by jaundice, nausea, and severe loss of appetite	Genetically engineered
* The last recorded case occurred in October 1977 in Somalia; the viral culture is stored in several laboratories.

Table 15.4. Some common bacterial diseases in humans (grouped according to their mode of transmission)

Name	Causative Agent	Mode of Transmission	Symptoms	Vaccine Type or Antibiotic
Diphtheria	Corynebacterium diphtheriae (Gram-positive bacillus)	Airborne droplet	Bacteria colonize the mucous membranes of the upper respiratory tract. Toxin is distributed by the bloodstream throughout the body. Mild fever, pharyngitis, followed by severe damage to The Heart, Nervous system, and Adrenal Glands	Toxoid
Tuberculosis	Mycobacterium tuberculosis (rod-shaped actinomycete)	Airborne droplet, drinking milk from infected cows	Bacteria can infect many Organs, but the lungs are most frequently affected. Weight loss, cough. Sputum may contain blood	BCG (attenuated bacterium). A tuberculin skin test is performed first to check for pre-existing Immunity. Streptomycin is the most commonly used antibiotic
Whooping cough (pertussis)	Bordetella pertussis (Gram-negative bacillus)	Airborne droplet	Primarily affects young children. Bouts of severe coughing accompanied by characteristic "whooping" inspirations due to airway narrowing	Inactivated bacterium
Gonorrhea	Neisseria gonorrhoeae (Gram-negative coccus — "gonococcus")	Sexual contact	Primarily affects the mucous membranes of the urogenital tract. In men — burning and discomfort during urination, followed by yellow discharge. Fever, headache, and general malaise. The Prostate Gland and Epididymis may become infected. Untreated infections can lead to Arthritis. In women, external genitalia are asymptomatic. The Urethra and cervix are primarily affected, from where the infection spreads to the Fallopian tubes, filling them with pus and leading to Infertility	Penicillin and streptomycin are the most commonly used Antibiotics
Syphilis	Treponema pallidum ("pale spirochete")	Sexual contact	A chronic, systemic infection. Incubation period 2–4 weeks. Initially, painless ulcers (hard chancres) appear on the body, healing within 3–8 weeks. After 6–8 weeks (secondary syphilis), fever develops and a skin rash appears. The patient is most infectious at this stage. Tertiary syphilis is no longer transmissible. The rash disappears, but many Internal Organs are damaged; heart disease, mental disorders, and blindness may occur	Penicillin is the most commonly used antibiotic
Typhus	Rickettsia spp. (intracellular parasites)	Epidemic (louse-borne): transmitted by lice. Endemic (murine): transmitted by the rat flea (spread among rats by fleas and lice)	12–14 days after infection: headache, backache, and limb pain. Measles-like rash in the armpits, on the hands and forearms. Delirium develops, followed by coma. Damage to blood vessel linings may occur, causing disseminated intravascular coagulation. Death results from toxemia, Heart Failure, or Kidney failure	Killed or live (avirulent) strain. Tetracyclines and chloramphenicol are the most commonly used antibiotics (vector control is also crucial)
Tetanus	Clostridium tetani (Gram-negative bacillus)	Wound infection	Bacterial toxins cause muscle spasms in the mouth and neck region, which then spread throughout the body. Convulsions become so severe that the patient cannot breathe and dies from oxygen deprivation	Toxoid
Botulism	Clostridium botulinum (anaerobic Gram-negative bacillus)	Ingestion of contaminated food	Vomiting, constipation, muscle paralysis, and intense thirst after 24 hours. Mortality rate is 50%	Toxins can be neutralized with antitoxins
Cholera	Vibrio cholerae (Gram-negative vibrio, i.e., a comma-shaped microbe)	Fecal-oral transmission: a) food or Water contaminated with patient feces; b) objects contaminated with patient feces; c) vectors such as flies that have come into contact with patient feces	Bacterial toxins cause intestinal inflammation leading to severe diarrhea, resulting in such massive water loss that the stool resembles "rice water." Death occurs due to dehydration and electrolyte loss	Killed bacteria: short-term and not always effective protection. A genetically engineered vaccine is available. Tetracycline and chloramphenicol are the most commonly used antibiotics
Typhoid fever	Salmonella typhi (= Salmonella typhosa) (Gram-negative bacillus)	Same as cholera	Mild fever, slight abdominal pain. The digestive tract is affected, with the infection spreading to the lymph and blood, lungs, Bone Marrow, and Spleen. Fever and pain intensify, and diarrhea begins. Intestinal ulceration and perforation are possible. Incubation period is 2–3 weeks	Polysaccharide derived from the bacterial capsule. A genetically engineered vaccine is available
Bacillary dysentery (shigellosis)	Shigella dysenteriae (Gram-negative bacillus)	Same as cholera	Bacterial toxins in the intestine cause abdominal pain with bloody, mucus-containing diarrhea. Symptoms appear 2–3 days after infection	No vaccine available. Tetracyclines are the most commonly used antibiotics
Bacterial food poisoning (gastroenteritis or salmonellosis)	Salmonella spp. (Gram-negative bacilli)	Mainly via contaminated meat (primarily pork and poultry). Fecal-oral transmission possible, as in cholera	The digestive tract is affected. Symptoms (abdominal pain, vomiting, and diarrhea) caused by bacterial toxins can develop rapidly and resolve quickly	No vaccine available. Tetracyclines are the most commonly used antibiotics, though they have limited efficacy and are not routinely prescribed

Table 15.5. Some common human diseases caused by Protozoa (protozoan infections)

Name	Causative Agent	Mode of Transmission	Symptoms	Prevention and Treatment
Malaria	Plasmodium spp.	Bites of mosquitoes of the genus Anopheles	Severe fever 10 days after the bite. High Temperature remains constant or occurs in paroxysms at varying intervals depending on the parasite species	Destruction of mosquito larvae by spraying water bodies with insecticides or oil aerosols. Drainage of vector breeding sites. Protective clothing and repellents. Individual Chemoprophylaxis mainly with chloroquine, and treatment most commonly with primaquine
Amebiasis (amebic dysentery)	Entamoeba histolytica	Fecal-oral transmission mainly via food and drinking water, less commonly through personal contact	Bloody diarrhea, fever, nausea, and vomiting. Fatal outcomes are possible	Observing hygiene during food preparation and consumption. Fly control. No acceptable chemoprophylaxis is known. Treatment primarily with metronidazole and diloxanide furoate
African trypanosomiasis (sleeping sickness in humans, nagana in cattle); transmitted between humans and domestic animals	Trypanosoma brucei	Tsetse fly bites	Swollen lymph nodes, fever, followed by enlargement of the spleen and liver. Later, the parasite infects The Nervous System, leading to drowsiness and muscle spasms	Tsetse flies inhabit a restricted geographic area in Africa where the disease is endemic. Protection against flies is necessary: window and door screens, spraying livestock; relocating people to safe areas. Treatment for humans is predominantly pentamidine

Table 15.6. Some common human diseases caused by Fungi (mycoses)

Name	Causative Agent	Mode of Transmission	Symptoms	Prevention and Treatment
Athlete's FOOT	Trichophyton mentagrophytes	Via damp floors in swimming pools, bathhouses, etc.	Macerated, scaly, cracking skin between the toes. Symptoms mainly appear and persist during hot months	Disinfection of floors in public baths, showers, and swimming pools. Avoiding contact with infected individuals. Treatment — oral griseofulvin (antibiotic)
Scalp ringworm (tinea capitis)	Trichophyton spp., Microsporum spp.	Highly contagious: via Hair contact with brushes, headwear, and similar objects used by an infected person	Initially small, gradually expanding scaly patches on the scalp with shedding and broken hairs. Scales are grayish, thicker at the edges of the patch, forming a distinct border	Topical application of antifungal ointments. Oral administration of the antibiotic griseofulvin
Candidiasis	Candida albicans (Yeast-like fungus)	Symptoms may occur on the skin, in the mouth, Vagina, intestines, etc. The causative agent is constantly present on the body but causes disease only under specific conditions, such as immune suppression or decreased environmental acidity (in the vagina during pregnancy, with diabetes, etc.). Infants are infected through contact with an infected mother (so-called "thrush" on the oral mucosa)	Red, inflamed, usually itchy patches on the body with discharge or whitish coating	Identifying and eliminating predisposing factors. Topical treatments — antifungal lotions, creams, or pessaries (for vaginal infections). Oral amphotericin

Incubation period — the time from infection to the appearance of the first symptoms of the disease.

Infectious period — the time during which an infected person can transmit the disease to healthy individuals.

Carrier — an individual who shows no symptoms of the disease, yet harbors the pathogen and is capable of transmitting it to healthy hosts.

Notifiable disease — a disease whose cases doctors are legally required to report to public health authorities due to the risk of an epidemic (e.g., cholera, tuberculosis, poliomyelitis).

Epidemic — the rapid spread of a disease among A large number of individuals followed by its subsequent decline.

Pandemic — an epidemic that spans an entire continent or even the entire globe.

Endemic — a disease that is constantly present in a certain proportion of the population within a given region.

Objective symptoms are signs of a disease that a physician can detect during a patient examination, such as a rash or a high temperature.

Subjective symptoms are signs of a disease identified by the patient themselves, such as a headache or nausea.

Symptomatology is the totality of subjective and objective symptoms.

Prophylaxis (prevention) refers to measures that prevent the onset of a disease, such as vaccination, adherence to hygiene, and sewage treatment.

Treatment comprises measures aimed at destroying the pathogen in the patient's body or alleviating symptoms, such as the administration of antibiotics.

15.3.1. Cholera

Cholera is a classic example of a water-borne disease. It is endemic to several regions in Asia and is particularly prevalent in India. From time to time, epidemics spread to other PARTS OF THE world, as was the case in Peru in 1992, when a cholera outbreak occurred in South America for the first time in the 20th century. In 1991, half a million cases of the disease were registered worldwide, 16,000 of which were fatal. Advances in treatment Methods have significantly reduced cholera mortality, but it remains a serious threat to humanity. Before the advent of effective antibiotics, this disease claimed a huge number of lives. For instance, during the epidemic of 1832, it killed half a million residents of New York City.

Routes of transmission and symptomatology

Cholera is caused by a curved, comma-shaped bacterium visible under a Microscope—the cholera bacterium (Vibrio cholerae). The main source of infection is water contaminated with the faeces of sick individuals or asymptomatic pathogen carriers. A typical clinical picture develops in about half of the people infected with the vibrio; the rest become bacterial carriers. The carrier state is rarely recognized; such individuals are not isolated from society, and their pathogen-containing faeces pose a particular danger to others. Drinking contaminated water, bathing in it, washing hands in it, or contaminating food and household items are the most common routes of infection. Direct contamination of food with faeces due to poor hygiene is also possible.

Although the cholera bacterium can survive in the environment, it multiplies exclusively in the human intestine, releasing a potent toxin that causes severe mucosal damage and watery diarrhea.

The primary symptom of cholera is diarrhea. The stool resembles rice-water in appearance. The body loses up to 15 liters of water a day. Abdominal pain and vomiting are also common. Fever is absent; on the box, the skin feels cold and clammy. In the absence of adequate therapy, rapid dehydration occurs, leading to the patient's death.

If hygiene rules are neglected—specifically, if faeces are not isolated from the environment by a sewage system and do not undergo subsequent neutralization treatment—a single patient can infect an entire settlement. One milliliter of their excrement contains up to 100 million cholera vibrios. Asymptomatic carriers or undiagnosed patients with mild symptoms also pose a major danger, as they can freely introduce the infection into new regions.

Treatment and prevention

The main cause of death from cholera is dehydration; therefore, the primary therapeutic measure is the replenishment of the body's water and mineral salts. In outbreak areas, such as refugee camps, a cheap and rapid method of oral rehydration is used—via a standard aqueous solution of salts and sugar (Fig. 15.4). The patient should drink one and a half times the volume of liquid faeces lost. Alternatively, the rehydration solution can be administered intravenously via drip.

Fig. 15.4. A UNICEF leaflet calling for donations and pointing out that a packet of salt mixture ready for dilution for a rehydration solution costs just 7 pence. These packets can be used not only for cholera, but also for severe diarrhea of any etiology. The caption at the top of the leaflet reads: "Oral rehydration salts save children during the Rwandan crisis."

Various antibiotics, such as tetracyclines and chloramphenicol, kill the pathogen and alleviate the symptoms of diarrhea. Chloramphenicol is effective against tetracycline-resistant strains (varieties) of the cholera vibrio.

In 1849, Dr. John Snow, seeking to prove that water was the cause of a devastating cholera epidemic in London, shut off the main water supply in a slum district (in plain terms, removed the handle from the water pump). The epidemic stopped immediately. However, the authorities were not convinced by this incident. During the next epidemic in 1854, Snow demonstrated that cholera victims were predominantly residents who drew water from the lower reaches of the Thames. Further upstream, where the river was cleaner, people barely fell ill. In 1875, the Public Health Act was passed, requiring the purification of sewage and drinking water, and by 1893, all cholera cases in Great Britain were caused by pathogens imported from abroad.

The Introduction of basic hygiene rules in all industrialized countries, including the Organization of household waste disposal in designated areas and the installation of flush toilets connected to a sewage system, made major cities much cleaner. By 1900, life expectancy in Britain had increased significantly, and the incidence of all infectious diseases (not just cholera) had declined.

Cholera is one of five epidemic diseases whose cases must be compulsorily reported to the WHO headquarters in Geneva. In response to incoming reports, international measures to prevent the spread of the infection are promptly implemented. These include the imposition of quarantine in the outbreak zone.

The key measures for combating cholera are as follows:

1. Providing people with clean drinking water.

2. Adequate treatment of municipal wastewater and maintaining cleanliness in populated areas.

3. Compliance with personal and public hygiene rules, especially when handling food (e.g., washing hands after using the toilet and before eating).

4. Sanitary and hygienic education of the population.

5. Vaccination recommended for travelers visiting or residing in cholera-endemic areas. The vaccine contains heat-killed vibrios. It is only 40–60% effective, and artificial immunity lasts for approximately 3–6 months. However, revaccination (booster dose) rapidly induces an immunological response and provides protection during an outbreak. Research is currently underway to develop a genetically engineered vaccine. This requires identifying and cloning the genes responsible for toxin synthesis. Another promising approach involves obtaining a low-virulence (attenuated) vibrio strain lacking one or two virulence genes.

6. Control of flies, which act as mechanical vectors transferring fecal matter to food.

7. Isolation of patients and heightened precautions when handling their feces and vomitus.

8. Identification of bacterial carriers and barring them from working in public catering establishments.

9. Immediate stool analysis of all diarrhea patients for the presence of the cholera vibrio, followed by the prompt administration of effective antibiotics upon detection.

10. Chemoprophylaxis for all close contacts of cholera patients to eliminate any pathogens that may have entered their bodies, along with the vaccination of residents in areas where an outbreak has been recorded.

In the 1970s, it was discovered that the cholera vibrio is capable of penetrating microscopic Algae and, by encysting within them, remaining dormant for months or even years. Since then, numerous cholera outbreaks have been reported in the coastal regions of India, South America, and Southeast Asia. These were found to be caused by a new strain of Vibrio cholerae, first identified in Indonesia in 1961. It exhibited higher virulence than the classical pathogen and, as it turned out, was able to survive in seawater and consequently migrate across the ocean between continents. This migration is facilitated by marine pollution from sewage discharges: the nutrients therein stimulate the growth of planktonic algae, which likely serve as the vehicle for the spread of the infection.

Last update: 06/08/2026

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