Disease Transmission

For a communicable disease to persist in a population, the pathogen must be able to move from one host to another. The route of transmission determines what measures can stop the disease — blocking that route can halt an epidemic in its tracks. OCR specification 4.1.1 (c) requires you to describe the means of transmission of communicable diseases in plants and animals, and the factors affecting transmission. This lesson covers both direct and indirect routes, with worked examples.

Key Definitions:

• Transmission — the passing of a pathogen from one host to another.
• Vector — an organism that carries a pathogen from one host to another without itself being affected.
• Reservoir — a host population in which the pathogen is maintained.
• Carrier — an individual infected with a pathogen but showing no symptoms, who can still transmit it.
• Fomite — an inanimate object that carries a pathogen (e.g., door handle, surgical instrument).

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Direct Transmission

Direct transmission is the movement of a pathogen from one host to another without an intermediate. It includes:

1. Direct Contact

Physical contact between infected and uninfected individuals — shaking hands, kissing, sexual intercourse. Pathogens transmitted this way include:

• HIV (via sexual contact and blood).
• Ringworm and athlete's foot (skin-to-skin contact).
• Streptococcus and Staphylococcus skin infections.

2. Faecal-Oral Route

Contaminated food or water containing faeces from an infected person infects the next host when ingested. This is the main route for:

• Cholera (Vibrio cholerae).
• Typhoid fever (Salmonella typhi).
• Rotavirus and norovirus (causes of gastroenteritis).

Good sanitation — flushing toilets, treated sewage, clean drinking water, handwashing — breaks this cycle.

3. Droplet Transmission

Pathogens are expelled in droplets when an infected person coughs, sneezes or speaks. Droplets travel short distances (up to ~1 m) and are inhaled by nearby individuals. Examples:

• Influenza
• Tuberculosis (larger droplets)
• Bacterial meningitis
• SARS-CoV-2

4. Airborne / Spore Transmission

Smaller particles (aerosols < 5 μm) remain suspended in air for longer and travel greater distances. In plants, many pathogens spread as spores carried on the wind — for example, the spores of Phytophthora infestans (potato blight) and Mycosphaerella fijiensis (black sigatoka).

5. Inoculation (Direct Entry Through Skin)

A puncture of the skin allows pathogens to enter the bloodstream directly — through wounds, bites, or contaminated needles. Examples include HIV (needlestick injuries), tetanus (Clostridium tetani via deep wounds) and rabies (animal bites).

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Indirect Transmission

Indirect transmission involves an intermediate between two hosts — either a vector or a fomite.

1. Vectors

A vector is another organism that carries the pathogen between hosts. Vector-borne diseases include:

• Malaria — Anopheles mosquitoes transfer Plasmodium between humans.
• Lyme disease — ticks transfer Borrelia burgdorferi.
• Dengue fever — Aedes mosquitoes transfer dengue virus.
• Plague — fleas transfer Yersinia pestis.

In plants, insects also act as vectors. Aphids, for example, transmit many plant viruses (including potato leafroll virus) as they feed.

2. Contaminated Food and Water

Water supplies contaminated with faeces can transmit waterborne diseases across whole populations (the classic example is John Snow's work on cholera at the Broad Street pump in 1854). Food can also carry pathogens — undercooked poultry is a major source of Salmonella and Campylobacter.

3. Fomites

Inanimate objects — tools, clothing, bedding, door handles, medical equipment — can carry pathogens between hosts. Hospital-acquired infections often spread via fomites, which is why strict disinfection is required.

flowchart TD
    A[Infected host] --> B{Direct?}
    B -->|Yes| C[Contact]
    B -->|Yes| D[Droplet]
    B -->|Yes| E[Faecal-oral]
    B -->|Yes| F[Spore/airborne]
    B -->|No| G[Vector]
    B -->|No| H[Fomite]
    B -->|No| I[Contaminated food/water]
    C --> Z[New host]
    D --> Z
    E --> Z
    F --> Z
    G --> Z
    H --> Z
    I --> Z

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Transmission in Plants