Module 4: Communicable Diseases
These free OCR A Level Biology Communicable Diseases revision notes have been written for specification points 4.1.1(a) and 4.1.1(b).
Communicable Diseases
Communicable diseases are caused by pathogens which invade a host and cause harm; they are infectious.
Pathogens are disease-causing microorganisms that damage the host’s cells and disrupt normal biological functioning.
Microorganisms can infect both animals and plants.
There are four main types of pathogenic microorganisms: bacteria, fungi, protoctists and viruses.
The table below summarises the key pathogens you need to know in OCR A level Biology, along with the diseases they cause and how they are transmitted; candidates have not been expected to recall the effects and symptoms of these different pathogens.
The table below gives two bacterial pathogens:
| Disease | Host | Transmission |
|---|---|---|
| Tuberculosis (TB) | Humans | Airborne droplets |
| Ring Rot | Potatoes, Tomatoes | Contact with contaminated tools, soil, or infected tubers |
The table below gives three viral pathogens:
| Disease | Host | Pathogen | Transmission Method |
|---|---|---|---|
| AIDS (Acquired Immune Deficiency Syndrome) | Humans | HIV: Human Immunodeficiency Virus |
– Bodily fluids (e.g. blood, semen). – Unprotected sex. – Needle sharing. |
| Influenza | Humans | Influenza virus |
– Airborne droplets. – Contaminated surfaces. |
| Tobacco Mosaic Virus | Plants (Tobacco, tomatoes) | TMV: Tobacco mosaic virus |
– Contact with contaminated tools, hands or other plants. – Insect vectors. – Soil. |
The table below gives two protoctista pathogens:
| Disease | Host | Transmission Method |
|---|---|---|
| Malaria | Humans | Vector transmission by female Anopheles mosquitoes. |
| Blight | Potatoes, Tomatoes |
Spores: Spread by the wind, rain and contaminated soil. |
The table below gives two fungal pathogens:
| Disease | Host | Transmission Method |
|---|---|---|
| Athlete’s Foot | Humans | Direct contact with contaminated floors, towels or communal showers. |
| Black Sigatoka | Bananas | Airborne spores that spread in warm and humid environments. |
Transmission
Transmission is how pathogens spread, and can be direct or indirect:
- Direct: The pathogen is transmitted from one organism to another; there is no intermediate organism.
- Indirect: The pathogen is transmitted from one organism to another via a vector; an intermediate organism carries the pathogen from an infected organism to an uninfected one.
Transmission in Animals
The table below outlines some of the ways animals can be infected by direct transmission:
| Method of Transmission | Description |
|---|---|
| Direct Contact | Physical contact between an infected and a healthy individual, or with contaminated surfaces. |
| Ingestion | Consuming food or drink contaminated by infected urine or faeces. |
| Droplet Infection | Pathogens are carried in droplets of mucus/saliva through the air when an infected person coughs or sneezes. |
| Spore Transmission | Spores in the air, soil, or on surfaces enter through cuts, inhalation, or wounds. |
The table below outlines some of the ways animals can be infected by indirect transmission:
| Method of Transmission | Description |
|---|---|
| Vector | Another organism (the vector) carries the pathogen between hosts. |
| Fomites* | Inanimate objects contaminated by pathogens which are passed between individuals (e.g. bedding, clothing, hospital equipment). |
*You do not need to know this term.
Transmission in plants
The table below outlines some of the ways plants can be infected by direct transmission:
| Method of Transmission | Description |
|---|---|
| Direct Contact | Contact between a healthy plant and an infected plant. |
| Soil Contamination | Pathogens in the soil infect healthy plants, and some remain viable for years. |
The table below outlines some of the ways plants can be infected by indirect transmission:
| Method of Transmission | Description |
|---|---|
| Vectors | Insects transfer pathogens between plants. |
| Water and Wind | Spores, bacteria, or viruses spread by the wind or movement of water (e.g. rain splashing, irrigation) |
| Human Activity | Pathogens spread by contaminated hands, tools, machinery, or transport of infected plant material. |
Social Factors In Transmission
Social factors influence how easily pathogens spread through a population. These factors generally increase transmission by:
- Increasing contact between individuals, such as in crowded or shared environments.
- Reducing sanitation and hygiene, allowing pathogens to persist on surfaces or in water.
- Enabling direct transmission routes, especially through behaviours or shared equipment.
- Delaying diagnosis and treatment allows infections to spread unchecked.
Prevention strategies target these risk points by reducing exposure, promoting hygiene, limiting shared contamination routes, and improving access to healthcare.