A pathogen is a microorganism that causes a disease. There are four main types of pathogen:

PathogenExample in animalsExample in plants
VirusesHIV potentially leading to AIDSTobacco mosaic virus
FungiAthlete's footRose black spot
ProtistsMalariaDowny mildew

All types of pathogen have a simple life cycle. They infect a host, reproduce themselves or replicate if it is a virus, spread from their host and infect other organisms. They also all have structural adaptations that make them successful at completing their life cycles, which enable them to cause further disease.

Diseases caused by pathogens are called communicable diseases. This means they can be transferred from one person to another.

There are other types of disease which cannot be caught:

  1. Inherited genetic disorders like cystic fibrosis.
  2. Deficiency diseases which are caused by a lack of essential vitamins or minerals, such as scurvy which occurs when an individual has insufficient vitamin C.
  3. Diseases like cancer that develop as a result of exposure to carcinogens or develop naturally as cell division occurs incorrectly.

All organisms are affected by pathogens. Even bacteria are infected by certain types of virus. Some of these infections can be transferred to organisms of a different species.


Transmission can occur in a number of important ways, as shown in the table below.

Direct contactThis can be sexual contact during intercourse or non-sexual contact, like shaking hands.
WaterDirty water can transmit many diseases, such as the cholera bacterium.
AirWhen a person who is infected by the common cold sneezes, they can spray thousands of tiny droplets containing virus particles to infect others.
Unhygienic food preparationUndercooked or reheated food can cause bacterial diseases like Escherichia coli which is a cause of food poisoning.
VectorAny organism that can spread a disease is called a vector. Many farmers think tuberculosis in their cattle can be spread by badgers.