Human Diseases - Notes, Biology

Bacterial Diseases

Tuberculosis

• Causative agent: Mycobacterium tuberculosis.
• Mode of infection: Primarily spread by airborne droplet nuclei produced when an infected person coughs or sneezes.
• Clinical features: Persistent cough (often productive), fever (commonly low-grade and with night sweats), weight loss, fatigue, chest pain and, in advanced disease, haemoptysis.
• Diagnosis: Chest X-ray, sputum smear microscopy, culture and molecular tests (e.g., GeneXpert). The Mantoux test uses a purified protein derivative called tuberculin to detect immune response; tuberculin is not a toxin produced by the bacterium.
• Prevention and control: BCG (Bacillus Calmette-Guérin) vaccine offers protection - especially against severe childhood forms such as miliary TB and TB meningitis - and public-health measures (case detection and effective treatment) reduce transmission.
• Treatment: Prolonged multi-drug antibiotic therapy under national TB programmes.

Diphtheria

• Causative agent: Corynebacterium diphtheriae.
• Age group: Most common in young children (around 2-5 years) but can affect older children and adults.
• Pathology and symptoms: The bacterium produces diphtheria toxin that causes a greyish pseudomembrane in the throat, sore throat, fever and can block the airway causing difficulty in breathing; systemic toxin effects can damage the heart and peripheral nerves.
• Prevention: Immunisation with DPT (Diphtheria, Pertussis, Tetanus) vaccine; primary immunisation starts in infancy (first dose commonly at about 6 weeks) followed by scheduled boosters.
• Treatment: Antitoxin and appropriate antibiotics; prompt management of airway obstruction where required.

Whooping Cough (Pertussis)

• Causative agent: Bordetella pertussis.
• Clinical course: Starts like a mild upper respiratory infection, then paroxysmal coughing fits with an inspiratory "whoop", vomiting after coughing and risk of hypoxia in infants.
• Prevention: Immunisation with DPT vaccine; boosters recommended in some schedules.
• Treatment: Macrolide antibiotics and supportive care; early antibiotic therapy reduces transmission.

Cholera

• Causative agent: Vibrio cholerae (historically called "vibrio comma").
• Mode of transmission: Faecal-oral route, usually through contaminated water or food.
• Clinical features: Profuse watery "rice-water" diarrhoea, vomiting, muscle cramps and rapid dehydration; severe cases can lead to hypovolaemic shock and death if untreated.
• Treatment: Immediate rehydration (oral rehydration salts, IV fluids when needed); antibiotics in severe cases.
• Prevention: Safe water and sanitation, food hygiene, health education and oral cholera vaccines where appropriate.

Vibrio cholerae 

Diarrhoea (Acute Intestinal Infections and Food Poisoning)

• Definition: Group of intestinal infections causing diarrhoea; may be bacterial, viral, protozoan or due to food toxins.
• Common causative bacteria: Escherichia coli, Shigella species, Campylobacter jejuni, Salmonella species.
• Transmission: Contaminated food and water, poor hygiene and sanitation.
• Symptoms: Loose or watery stools, abdominal pain, vomiting and possible fever; risk of dehydration, especially in young children and the elderly.
• Management and prevention: Rehydration therapy (ORS), zinc supplements in children, antibiotics only when indicated, improved sanitation and food safety.

Leprosy (Hansen's Disease)

• Causative agent: Mycobacterium leprae.
• Pathology and symptoms: Chronic infection primarily of skin and peripheral nerves causing skin lesions, loss of sensation, nodules, ulcers and deformities of hands and feet.
• Transmission: Requires prolonged close contact; relatively low infectivity compared with many other infectious diseases.
• Treatment: Multi-drug therapy (MDT) recommended by WHO (e.g., dapsone, rifampicin and clofazimine).

Tetanus

• Causative agent: Clostridium tetani, a motile, spore-forming bacterium commonly present in soil, dust and animal feces.
• Mode of entry: Through wounds, burns or puncture sites contaminated with spores.
• Incubation: Usually about one to two weeks but may vary.
• Clinical features: Painful, sustained muscle contractions and spasms, especially of jaw and neck muscles (hence "lockjaw"), progressing to respiratory muscle involvement; the disease affects the central nervous system.
• Treatment: Human tetanus immunoglobulin or antitoxin (ATS/antitetanus serum), wound care, muscle relaxants and supportive care.
• Prevention: Active immunisation with tetanus toxoid (part of DPT/DT boosters) and proper wound care.

Pneumonia

• Causative agent: Many organisms can cause pneumonia; a common bacterial cause is Streptococcus pneumoniae.
• Definition: Inflammatory infection of the lungs causing consolidation of lung tissue and accumulation of fluid in alveoli and bronchioles, impairing gas exchange.
• Symptoms: Sudden chills, high fever, chest pain, productive cough, rapid shallow breathing and difficulty breathing.
• Treatment and prevention: Appropriate antibiotics, oxygen and supportive care; pneumococcal vaccines reduce risk in vulnerable groups.

Typhoid

• Causative agent: Salmonella enterica serotype Typhi (commonly referred to as Salmonella typhi).
• Transmission: Faecal-oral route through contaminated food, water or via flies and poor personal hygiene.
• Symptoms: Prolonged high fever, headache, abdominal pain, diarrhoea or constipation, and sometimes intestinal ulceration; rose spots on the trunk may be seen.
• Prevention: Vaccination, safe water, sanitation and hand hygiene.
• Treatment: Appropriate antibiotics guided by resistance patterns and clinical severity.

Plague

• Causative agent: Yersinia pestis (historically listed as Pasteurella pestis).
• Primary hosts and vector: Wild and commensal rodents (rats) are primary hosts; the rat flea Xenopsylla cheopis is the principal vector. Human infection can also occur via lice or direct contact in some circumstances.
• Clinical forms: Bubonic plague (bacteria multiply in blood and lymph nodes causing painful swollen lymph nodes called buboes), pneumonic plague (involves lungs and can spread person-to-person by respiratory droplets), and septicemic plague (bacterial multiplication in blood leading to severe sepsis).
• Historic note: Severe epidemics of plague (the "Black Death") caused massive mortality in medieval Europe.
• Treatment and control: Prompt antibiotic therapy, rodent control and flea control reduce transmission; pneumonic plague requires urgent isolation and treatment.

Anthrax

• Causative agent: Bacillus anthracis.
• Hosts and transmission: Common in herbivorous animals (cattle, sheep, goats); humans acquire infection from contact with infected animals, contaminated animal products or by inhalation of spores.
• Clinical forms: Cutaneous anthrax (most common), inhalational anthrax (most severe), and gastrointestinal anthrax.
• Prevention and treatment: Animal vaccination, safe handling of animal products, antibiotics and supportive care for humans; anthrax spores require special decontamination measures.

Gonorrhoea

• Causative agent: Neisseria gonorrhoeae.
• Mode of transmission: Sexual contact.
• Complications: Can cause pelvic inflammatory disease in women and may lead to infertility if untreated; neonatal infection at birth is possible.
• Treatment: Antibiotics; note rising antimicrobial resistance is an important concern.

Syphilis

• Causative agent: Treponema pallidum.
• Mode of transmission: Sexual contact and from mother to foetus (congenital syphilis).
• Clinical course: Multiple stages (primary chancre, secondary rash and mucous lesions, latent stage, tertiary disease affecting heart and nervous system) if untreated.
• Treatment: Penicillin remains the treatment of choice.

Viral Diseases

Mumps

• Causative agent: A paramyxovirus.
• Clinical features: Painful swelling of salivary glands (especially the parotid), fever and difficulty opening the jaw. In post-pubertal males and females, the gonads (testes or ovaries) may become inflamed (orchitis/oophoritis).
• Prevention: MMR (Measles-Mumps-Rubella) vaccine provides effective protection.

Measles

• Causative agent: Measles (rubeola) virus.
• Incubation and early signs: Incubation about 10-14 days; early signs include fever, cough, coryza and conjunctivitis. Koplik spots on the buccal mucosa may appear before the skin rash.
• Symptoms: Characteristic maculopapular rash, high fever and complications such as pneumonia or encephalitis in some cases.
• Prevention: MMR vaccine; high coverage is important to prevent outbreaks.

Poliomyelitis

• Causative agent: Poliovirus, an enterovirus in the Picornaviridae family.
• Transmission: Faecal-oral route through contaminated food and water; virus multiplies in the intestinal mucosa and may spread via the bloodstream to the central nervous system.
• Pathology and symptoms: Virus preferentially infects and destroys anterior horn cells of the spinal cord, causing acute flaccid paralysis, often of the legs; severe cases lead to respiratory muscle paralysis.
• Prevention: Vaccination with oral polio vaccine (OPV) or inactivated polio vaccine (IPV) has been central to global polio control and eradication efforts.

Influenza (Flu)

• Causative agent: Influenza viruses of the family Orthomyxoviridae (types A and B cause most human disease).
• Transmission: Person-to-person by droplet infection from coughing or sneezing, or by contact with contaminated surfaces.
• Symptoms: Sudden fever, chills, sore throat, muscle aches, cough and nasal congestion; complications include viral or secondary bacterial pneumonia in high-risk groups.
• Prevention: Annual vaccination (strain composition updated each year), respiratory hygiene and antiviral drugs in certain cases.

Influenza virus

Common Cold

• Causative agent: Most commonly rhinoviruses (members of the Picornaviridae family), though many other viruses can cause common cold symptoms.
• Transmission: Droplet spread and contact with nasal and pharyngeal secretions.
• Symptoms: Nasal congestion, sneezing, watery eyes, sore throat, mild fever and cough; typically self-limited.
• Management: Symptomatic treatment, rest and fluids; antibiotics are not indicated for viral colds.

Rabies (Hydrophobia)

• Causative agent: Rabies virus (a Lyssavirus), a neurotropic RNA virus.
• Hosts and transmission: Affects all warm-blooded animals; transmitted almost always by the bite of an infected animal (commonly dogs in many regions).
• Clinical features: Progressive encephalitis, agitation, hydrophobia (fear of water due to painful spasms of throat muscles), hypersalivation and coma; once clinical signs appear, disease is nearly always fatal.
• Prevention: Thorough wound cleaning and prompt post-exposure prophylaxis with rabies vaccine and, when indicated, rabies immunoglobulin. Louis Pasteur developed the first rabies vaccine.

Hepatitis

• Definition: Viral infection causing inflammation of the liver; clinical features include loss of appetite, nausea, dark urine, pale stools and jaundice due to raised bilirubin from liver cell injury.
• Hepatitis A (epidemic jaundice): Caused by hepatitis A virus (HAV); transmitted by the faecal-oral route; usually acute and self-limited.
• Hepatitis B (serum hepatitis): Caused by hepatitis B virus (HBV); transmitted by blood and body fluids (including unsafe injections, transfusions and perinatal transmission); may become chronic and lead to cirrhosis or hepatocellular carcinoma.
• Prevention: HAV and HBV vaccines (HBV is part of many childhood immunisation schedules), safe injection and blood-transfusion practices and hygiene measures.

Dengue Fever

• Causative agent: Dengue virus (a Flavivirus) with four primary serotypes.
• Vector: Female Aedes aegypti mosquito that bites mainly during daytime.
• Clinical forms: Classical dengue (high fever, severe headache, retro-orbital pain, severe myalgia/backache and mild conjunctivitis) and dengue haemorrhagic fever/dengue shock syndrome (severe, potentially fatal forms with plasma leakage, bleeding and shock).
• Management: Supportive care with careful fluid management; close monitoring for warning signs of severe disease.
• Prevention: Vector control, removal of mosquito breeding sites, personal protection from mosquito bites and targeted vaccination policies where recommended.

Yellow Fever

• Causative agent: Yellow fever virus (a Flavivirus).
• Transmission: Transmitted by infected Aedes mosquitoes in urban cycles and by other mosquito species in jungle cycles.
• Symptoms: Fever, headache, vomiting and jaundice from liver involvement; severe cases may show haemorrhage and multi-organ failure.
• Prevention: Efficient live attenuated vaccine gives long-lasting protection; mosquito control also important.

Protozoan Diseases

Malaria

• Causative agents: Protozoa of the genus Plasmodium; several species infect humans, including Plasmodium vivax, Plasmodium malariae, Plasmodium falciparum and Plasmodium ovale.
• Vector: Female Anopheles mosquitoes transmit the parasite.
• Clinical features: After an incubation period there are periodic febrile paroxysms with chills, high fever and sweating; the periodicity (every 48 or 72 hours) varies with the Plasmodium species. Severe disease (especially with P. falciparum) can cause cerebral malaria and death.
• Treatment: Antimalarial drugs (choice depends on species and resistance patterns); historically quinine (from the Cinchona tree) was used, and modern treatment includes artemisinin-based combination therapies (ACTs) for falciparum malaria.
• Prevention: Mosquito control, insecticide-treated nets, indoor residual spraying and chemoprophylaxis where appropriate; biological predators (certain fish) may consume mosquito larvae but are not substitutes for public-health measures.

Kala-azar (Visceral Leishmaniasis)

• Causative agent: Leishmania donovani.
• Vector: Sandfly (Phlebotomus species).
• Clinical features: Prolonged fever, marked enlargement of spleen and liver (splenomegaly and hepatomegaly), weight loss and anaemia.
• Other names: Kala-azar or dum-dum fever.
• Treatment: Specific antileishmanial drugs (e.g., antimonials or newer alternatives) and public-health vector control measures.

Giardiasis

• Causative agent: Giardia intestinalis (also called Giardia lamblia).
• Site of infection: Upper small intestine (duodenum and jejunum).
• Symptoms: Abdominal discomfort, mild diarrhoea with pale, bulky, foul-smelling and greasy stools (steatorrhea) and malabsorption.
• Transmission and prevention: Faecal-oral route via contaminated water or food; improved sanitation and water treatment prevent infection.

Amoebiasis (Amoebic Dysentery / Enteritis)

• Causative agent: Entamoeba histolytica.
• Clinical features: Abdominal pain, alternating diarrhoea and constipation, stools with blood and mucus, and possible formation of intestinal ulcers; extra-intestinal spread can cause liver abscess.
• Transmission: Faecal-oral route through contaminated water, food or hands.
• Treatment: Specific antiprotozoal medications and improved sanitation.

Helminthic Diseases

Ascariasis

• Causative agent: Ascaris lumbricoides (a roundworm).
• Transmission: Ingestion of embryonated eggs from contaminated soil, food or hands; flies and cockroaches can mechanically carry eggs but are not the biological vector.
• Symptoms: Often asymptomatic or cause abdominal pain, malnutrition, anaemia, intestinal obstruction in heavy infections.
• Treatment and prevention: Anthelmintic drugs, improved sanitation and hygiene.

Filariasis (Lymphatic Filariasis)

• Causative agent: Wuchereria bancrofti (others include Brugia malayi).
• Vector: Transmitted by mosquitoes (various genera such as Culex, Anopheles or Aedes depending on region).
• Clinical features: Fever, lymphangitis and progressive lymphatic obstruction leading to severe swelling of limbs or genitalia (elephantiasis).
• Prevention and control: Vector control, mass drug administration in endemic areas and personal protection from mosquito bites.

Taeniasis

• Causative agent: Taenia solium (pork tapeworm) and Taenia saginata (beef tapeworm); Taenia solium is especially important due to risk of cysticercosis.
• Mode of transmission: Ingestion of undercooked infected pork (T. solium) or beef (T. saginata); T. solium eggs can also infect humans leading to cysticercosis if ingested.
• Symptoms: Abdominal discomfort, indigestion, anorexia or neurological complications in cysticercosis (if larval cysts develop in the brain).
• Prevention: Proper cooking of meat, good sanitation and food hygiene.

Fungal Diseases

Ringworm and Other Dermatophytoses

• Causative agents: Dermatophyte fungi such as Microsporum, Trichophyton and Epidermophyton species.
• Clinical features: Superficial skin infections producing circular, red, scaly lesions (ring-shaped), itchy patches or blisters; infections may affect scalp, body, groin or feet.
• Transmission: Direct contact with infected persons, animals (e.g., unbathed cats or dogs) or fomites such as combs and clothing.
• Treatment: Topical antifungal agents for limited disease; systemic antifungals for extensive or scalp infections.

Other Important Fungal Infections

• Aspergillosis: Caused by Aspergillus species (e.g., Aspergillus fumigatus), can cause allergic disease, localized infections or invasive disease in immunocompromised persons.
• Cryptococcal meningitis: Caused by Cryptococcus neoformans, an important cause of meningitis in immunocompromised patients.
• Tinea pedis (Athlete's foot): Dermatophyte infection of the feet, commonly caused by species in the Trichophyton group.
• Dermatitis of beard and hair: Some dermatophytes (e.g., Trichophyton verrucosum) cause infections in facial hair and scalp.
• Prevention and control: Good personal hygiene, avoiding sharing personal items, prompt treatment of infected animals and environmental measures where indicated.

Summary: This chapter summarises major human infectious diseases grouped by causative organisms - bacteria, viruses, protozoa, helminths and fungi. For each disease the causative agent, typical mode of transmission, main clinical features, prevention and usual lines of management have been provided. Public-health measures (vaccination, sanitation, vector control, safe water and hygiene) alongside timely diagnosis and appropriate therapy are essential to control these diseases.