# Pathogen

A microorganism that causes disease. The main types are bacteria, viruses, fungi and protists.

Biology (school) · Part 04: Health and disease · Card 1 of 10 · Last updated 2026-10-19

## Analogy

An unwanted guest who damages the house.

## In practice

Influenza is caused by a virus. Salmonella food poisoning is caused by bacteria.

## Practice

### Flashcards

- **Name the four types of pathogen.** Bacteria, viruses, fungi and protists.

## Questions people ask

### What is the difference between bacteria and viruses?

Bacteria are living single cells that can reproduce by themselves and are killed by antibiotics. Viruses are much smaller, are not made of cells and can reproduce only inside a host cell. Antibiotics have no effect on viruses, which is why colds and flu need different treatments.

### Are all bacteria harmful?

No. Most bacteria are harmless and many are helpful. Bacteria in your gut help digest food and make some vitamins. Others are used to make yoghurt and cheese. Only a small number cause disease. Pathogenic bacteria release toxins or damage cells as they multiply.

## Related concepts

- [Communicable disease](https://studyvisually.com/learn/science/biology-school/biology-school-part-4/biology-school-part-4-concept-2)
- [Vector](https://studyvisually.com/learn/science/biology-school/biology-school-part-4/biology-school-part-4-concept-3)
- [Non-communicable disease](https://studyvisually.com/learn/science/biology-school/biology-school-part-4/biology-school-part-4-concept-10)

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