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

Example in animalsExample in plants
VirusesHIV/AIDSTobacco mosaic virus
FungiAthlete's footBlack rose 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 more disease.

Diseases caused by pathogens are called communicable diseases. This means the disease can be transferred from an infected organism to another organism.

There are other types of disease which cannot be passed from organism to organism. These are called non-communicable diseases:

  1. Inherited genetic disorders like cystic fibrosis can be passed to offspring but not to anyone else.
  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.


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

Type of transmissionExample
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.