1. Bacteria - Produce toxins that damage body cells.
2. Viruses - Use host cells to replicate before bursting out and destroying cells.
3. Protoctists (protists) - Take over cells and break them open.
4. Fungi - Digest living cells to destroy them. Some also produce toxins.

Direct transmission

Pathogens can be transferred directly from one organism to another.

This transfer can take place in several ways:

• Direct contact - Skin-to skin contact, kissing, or sexual intercourse.
• Airborne droplets - Coughing or sneezing tiny droplets of mucus or saliva onto someone.

Indirect transmission

Communicable diseases can also be passed from one organism to another using an intermediate (e.g. water or another organism).

This transfer can take place in several ways:

• Food and drinking water - Ingestion of contaminated food or water can cause disease.
• Vectors - These transmit pathogens from one host to another (e.g. mosquitoes transmit malaria).
• Contaminated objects - Pathogens from infected individuals can live on objects for a short time and infect others.

The risk of communicable disease is increased by certain factors:

• Living conditions - Overcrowded areas increase the risk of direct transmission.
• Climate - For example, warmer temperatures allow mosquitoes to breed and transmit malaria.
• Social factors - For example, a lack of health education and healthcare systems increase the risk of communicable disease.