Pathogens and How They Spread

A pathogen is a microorganism that causes disease. In this lesson you will learn about the four types of pathogen, how they cause disease, how they spread from one organism to another, and how we can reduce the spread of infection.

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What Is a Pathogen?

A pathogen is a microorganism that enters the body and causes disease. Pathogens include:

• Bacteria
• Viruses
• Fungi
• Protists

Not all microorganisms are pathogens. Many bacteria are harmless or even beneficial (e.g. gut bacteria that help with digestion). Only those that cause disease are called pathogens.

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The Four Types of Pathogen

1. Bacteria

• Single-celled prokaryotic organisms (no nucleus)
• Reproduce rapidly by binary fission (one cell splits into two)
• Cause disease by releasing toxins — poisonous chemicals that damage cells and tissues
• Some bacteria are beneficial; only a small proportion are pathogenic
• Size: typically 1–5 μm

Examples: Salmonella, Neisseria gonorrhoeae, Mycobacterium tuberculosis

2. Viruses