What Are Communicable (Infectious) Diseases?

Communicable diseases are infectious diseases that can spread between organisms. They are caused by pathogens such as viruses, bacteria, protists and fungi.

This content applies to both Foundation and Higher tiers. It is part of AQA GCSE Biology and is also shared with Combined Science.

What is a pathogen?

A pathogen is a microorganism that causes infectious disease. Pathogens can infect plants as well as animals. The four types required for AQA GCSE Biology are:

• Viruses, such as the measles virus, HIV and tobacco mosaic virus (TMV)
• Bacteria, such as those causing salmonella food poisoning and gonorrhoea
• Protists, such as the pathogen causing malaria
• Fungi, such as the pathogen causing rose black spot

Bacteria may reproduce rapidly inside the body and produce toxins. These are poisons that damage tissues and make us feel ill. Viruses reproduce inside living cells, damaging the cells as they do so.

How do communicable diseases spread?

Pathogens can spread in several ways:

• Through the air: measles spreads in droplets released by coughs and sneezes.
• Through water: some pathogens can be carried in contaminated water.
• By direct contact: gonorrhoea can spread through sexual contact.
• Through contaminated food: salmonella bacteria may be ingested in contaminated food.
• Using a vector: malaria is spread by mosquitoes. A vector is an organism that carries a pathogen between hosts.

Worked example

A person with measles coughs near someone who has not been vaccinated. Droplets containing the virus are inhaled by the second person. The measles virus then reproduces inside their cells, so the disease has been transmitted.

Common exam mistake

Do not say that all diseases are communicable. Non-communicable diseases, such as cancer, cannot be passed between organisms. Also, do not confuse the pathogen with the disease: HIV is the virus, while AIDS is the late stage of HIV infection.