explain how cholera, malaria, TB and HIV are transmitted

Infectious Diseases – Transmission (Cambridge International AS & A Level Biology 9700, Topic 10)

This set of notes covers the four core diseases required by the syllabus. For each disease the pathogen is identified, its taxonomic classification is given, the complete transmission pathway is described, the main biological, social and economic risk factors are linked to the route of transmission, and the principal public‑health control measures are listed.

1. Cholera

Pathogen & classification

• Vibrio cholerae – Gram‑negative, comma‑shaped bacterium; oxidase‑positive, motile by a single polar flagellum. Belongs to the family Vibrionaceae.

Primary transmission route (faecal‑oral)

1. Infected individuals excrete huge numbers of bacteria in their faeces.
2. Faeces contaminate water, food, or hands when sanitation is inadequate.
3. Susceptible persons ingest the bacteria by:

   • drinking contaminated water,
   • eating raw/undercooked shellfish that have filtered the water,
   • hand‑to‑mouth transfer after handling contaminated objects.

Epidemiological / risk factors

• Poverty‑related lack of sanitation: open defecation, poorly maintained latrines.
• Unsafe water supplies: surface water, shallow wells, storage tanks that are not chlorinated or filtered.
• Overcrowding & rapid population movement: accelerates spread once a water source is contaminated.
• Climate‑related flooding: floods disperse contaminated faeces over large areas, increasing exposure.

Control & prevention

• Provision of safe drinking water – chlorination, filtration, boiling.
• Improved sanitation – construction of latrines, sewage treatment, safe disposal of faeces.
• Health education on hand‑washing, safe food handling and water storage.
• Oral rehydration solution (ORS) for all cases; antibiotics (e.g., doxycycline, azithromycin) for severe disease.
• Oral cholera vaccine (e.g., killed whole‑cell or live‑attenuated) for high‑risk populations.

2. Malaria

Pathogen & classification

• Protozoan parasites of the genus Plasmodium (phylum Apicomplexa, obligate intracellular parasites). Species infecting humans:

  P. falciparum, P. vivax, P. ovale, P. malariae, P. knowlesi.

Complete transmission pathway (vector‑borne)

1. Human → mosquito: An infected person carries mature gametocytes in peripheral blood.
2. A female Anopheles mosquito takes a blood meal and ingests the gametocytes.
3. In the mosquito mid‑gut, gametocytes develop into male and female gametes, fuse to form a zygote, and become a motile ookinete.
4. The ookinete penetrates the gut wall and forms an oocyst on the outer surface of the mid‑gut.
5. Inside the oocyst, thousands of sporozoites develop; the oocyst ruptures, releasing sporozoites into the haemocoel.
6. Sporozoites migrate to the salivary glands of the mosquito.
7. Mosquito → human: During the next bite, sporozoites are injected with saliva into the dermis, enter the bloodstream and travel to the liver.
8. Hepatic (pre‑erythrocytic) stage: Sporozoites invade hepatocytes, develop into schizonts, and release thousands of merozoites into the blood.
9. Erythrocytic (blood) stage: Merozoites invade red blood cells, undergo asexual replication (ring → trophozoite → schizont), and cause the clinical symptoms of malaria when infected RBCs rupture.
10. Some merozoites differentiate into gametocytes, completing the cycle when taken up by another mosquito.

Epidemiological / risk factors

• Presence of competent vectors: Stagnant water, rice paddies, poorly drained areas provide breeding sites for Anopheles mosquitoes.
• Tropical/sub‑tropical climate: Warm, humid conditions extend mosquito lifespan and parasite development.
• Insufficient vector control: Lack of insecticide‑treated nets (ITNs) or indoor residual spraying (IRS) increases bite exposure.
• Poor housing: Open eaves, unscreened windows, and thatched roofs allow mosquito entry.

Control & prevention

• Insecticide‑treated bed nets (ITNs/LLINs) and indoor residual spraying (IRS) with WHO‑approved insecticides.
• Environmental management – removal of standing water, larviciding, drainage of breeding sites.
• Chemoprophylaxis for travellers (e.g., atovaquone‑proguanil, doxycycline, mefloquine).
• Prompt diagnosis (RDTs, microscopy) and treatment with artemisinin‑based combination therapy (ACT).
• Vaccination (RTS,S/AS01) in selected high‑risk areas – an additional tool, not a replacement for vector control.

3. Tuberculosis (TB)

Pathogen & classification

• Mycobacterium tuberculosis – Acid‑fast, aerobic, non‑sporing bacillus; order Actinomycetales, family Mycobacteriaceae.
• Related species of epidemiological importance:

  • Mycobacterium bovis – causes bovine TB and can infect humans (especially via unpasteurised milk).

Primary transmission route (airborne droplet nuclei)

1. People with active pulmonary TB expel aerosolised droplet nuclei (≤5 µm) when coughing, sneezing, speaking loudly, singing or laughing.
2. Droplet nuclei remain suspended in the air for minutes to hours, depending on ventilation.
3. A susceptible person inhales the nuclei; bacilli reach the alveoli, are phagocytosed by macrophages and may establish a primary infection.

Epidemiological / risk factors

• Overcrowded, poorly ventilated indoor settings: prisons, shelters, crowded households.
• Immunocompromised hosts: HIV infection, malnutrition, diabetes mellitus, chronic renal disease.
• Socio‑economic deprivation: limited access to health services delays diagnosis and treatment, prolonging infectiousness.
• Drug resistance: previous inadequate treatment can lead to multidrug‑resistant (MDR‑TB) and extensively drug‑resistant (XDR‑TB) strains.

Control & prevention

• Early case detection – sputum smear microscopy, rapid molecular tests (GeneXpert MTB/RIF).
• Standardised short‑course chemotherapy (DOT – Directly Observed Treatment, Short‑course); second‑line regimens for MDR/XDR‑TB.
• Improved ventilation of indoor spaces; ultraviolet germicidal irradiation where feasible.
• BCG vaccination of newborns in high‑incidence countries (protects against severe childhood TB).
• Contact tracing and prophylactic treatment (e.g., isoniazid) of close contacts.

4. Human Immunodeficiency Virus (HIV)

Pathogen & classification

• Human Immunodeficiency Virus (HIV) – Enveloped, single‑stranded RNA retrovirus; family Retroviridae, genus Lentivirus.

Primary transmission routes (body‑fluid exchange)

1. Sexual transmission: Unprotected vaginal or anal intercourse – virus present in semen, pre‑ejaculate, cervical and vaginal secretions.
2. Parenteral (blood‑borne) transmission: Sharing contaminated needles, transfusion of infected blood, or occupational needle‑stick injuries.
3. Perinatal (mother‑to‑child) transmission:

   • In utero – transplacental passage of virus.
   • During delivery – exposure to maternal blood and genital secretions.
   • Post‑natal – breastfeeding.

4. Accidental exposure: Contact of mucous membranes or non‑intact skin with infected fluid (rare in the community, more relevant to health‑care workers).

Epidemiological / risk factors

• Unprotected sexual activity: Multiple partners, low condom use, presence of other sexually transmitted infections.
• Injection drug use: Sharing needles or other injecting equipment.
• Unsafe blood services: Inadequate screening of donated blood or blood products.
• Poor antenatal care: Missed opportunities for maternal HIV testing and prophylaxis.
• Stigma and discrimination: Discourages testing, treatment uptake and open discussion of risk behaviours.

Control & prevention

• Consistent and correct use of male and female condoms.
• Needle‑exchange programmes and supervised injection facilities.
• Rigorous screening of all blood products and organ donations.
• Pre‑exposure prophylaxis (PrEP) for high‑risk groups and post‑exposure prophylaxis (PEP) after occupational accidents.
• Maternal HIV testing, antiretroviral therapy (ART) during pregnancy, safe delivery practices, provision of ART to the infant, and avoidance of breastfeeding where safe alternatives exist.
• Universal access to ART for all diagnosed individuals – suppresses viral load and dramatically reduces onward transmission.

Summary Comparison Table

Quick‑scan Checklist (Cambridge 9700 – Topic 10)

Key Points for Revision

1. Identify the pathogen and its taxonomic group (bacterium, protozoan, acid‑fast bacillus, virus).
2. Describe the full transmission pathway, including any vector or intermediate host stages.
3. Link each major risk factor to the way the disease spreads.
4. Recall the specific control or prevention strategy that targets the identified route (e.g., water treatment for cholera, ITNs for malaria, ventilation for TB, condom use for HIV).
5. Use the summary table to compare diseases quickly before the exam.