Pathogen - GCSE Biology Definition

A pathogen is a microorganism, such as a bacterium, fungus, protist, or a virus, that can cause disease in living organisms. These pathogens are usually too small to be seen without a microscope but can have a big impact on health by causing illness. They can spread in various ways, such as through the air, water, direct contact, or contaminated surfaces. The body’s immune system works to defend against these harmful microbes, but sometimes pathogens can overcome these defences and cause disease. Understanding pathogens in GCSE Biology helps explain how infectious diseases spread and how they can be prevented or treated.