Pathogens are disease-causing viruses, bacteria, fungi or protists, which can infect animals and plants. Humans have an immune system, which can defend them from pathogens.
A pathogen is a microorganism that causes a disease. There are four main types of pathogen:
| Pathogen | Example in animals | Example in plants |
|---|---|---|
| Viruses | HIV potentially leading to AIDS | Tobacco mosaic virus |
| Bacteria | Salmonella | Agrobacterium |
| Fungi | Athlete's foot | Rose black spot |
| Protists | Malaria | Downy 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:
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.
| Type | Examples |
|---|---|
| Direct contact | This can be sexual contact during intercourse or non-sexual contact, like shaking hands. |
| Water | Dirty water can transmit many diseases, such as the cholera bacterium. |
| Air | When 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 preparation | Undercooked or reheated food can cause bacterial diseases like Escherichia coli which is a cause of food poisoning. |
| Vector | Any organism that can spread a disease is called a vector. Many farmers think tuberculosis in their cattle can be spread by badgers. |