Communicable (infectious) diseases: Pathogens are microorganisms that cause infectious disease. Pathogens may be viruses, bacteria, protists or fungi. They may infect plants or animals and can be spread by direct contact, by water or by air.

The idea

A pathogen is a microorganism that causes infectious disease. The four main groups are viruses, bacteria, protists and fungi. Viruses reproduce inside host cells and damage them; bacteria multiply in tissues and may release toxins; protists are often single-celled and may be carried by vectors; fungi can grow on surfaces and penetrate tissues. Pathogens infect both plants and animals, for example tobacco mosaic virus in plants and influenza virus in humans. Spread occurs by direct contact, by water or by air. Direct contact includes touching, body fluids and contaminated surfaces. Water-borne spread occurs when pathogens enter drinking water or are carried in water droplets. Airborne spread occurs in droplets and aerosols released by coughing or sneezing.

What earns the marks

• Define a pathogen as a microorganism that causes infectious disease, distinguishing it from non-infectious causes such as diet or genetics.
• Name the four pathogen groups: viruses, bacteria, protists and fungi, and give one example of each.
• State that pathogens can infect both plants and animals, with named examples such as tobacco mosaic virus in plants and influenza virus in humans.
• Describe direct contact spread, including touching, body fluids and contaminated surfaces.
• Describe water-borne spread, including drinking contaminated water and water carrying pathogens.
• Describe airborne spread, including droplets and aerosols released by coughing or sneezing.

Common mistakes and how to put them right

• Calling all microorganisms pathogens.

  Put it right: Most microorganisms are harmless or useful, and only some cause disease.

• Saying bacteria are viruses or treating the groups as identical.

  Put it right: Viruses reproduce inside host cells, bacteria multiply outside cells and may release toxins.

• Forgetting that plants get diseases too.

  Put it right: Include a plant example such as tobacco mosaic virus alongside an animal example.

Exam tips

• Learn one named example for each pathogen group so you can illustrate the definition quickly.
• When asked how a disease spreads, name the route and give a concrete situation, such as droplets from a cough entering the air.
• Use precise terms: infectious disease, pathogen, host, vector and transmission, rather than vague words like 'germs'.