Pathogens and their transmission

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

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

Pathogens infect a host, reproduce themselves, or replicate if it is a virus, spread from their host and infect other organisms.

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

The spread of communicable disease

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

Direct contactThis can be through contact with other people, eg by shaking hands, or contact with surfaces that other people have touched. The common cold can be transmitted in this way. Pathogens can also be spread through sexual contact, eg the bacterium that causes gonorrhoea.
WaterThrough water that is contaminated with microorganisms, such as the cholera bacterium.
AirThrough the air when people cough or sneeze. When a person who is infected by the common cold, influenza (flu) or measles, sneezes, they can spray thousands of tiny droplets containing virus particles to infect others.
Contaminated foodThrough bacteria such as Salmonella in contaminated food. Food can be contaminated with bacteria by contact with infected people, or when it is prepared in unhygienic conditions.
Other animalsThrough animals that scratch, bite or draw blood.
VectorAny organism that can spread a disease is called a vector. Mosquitoes are the vectors in the spread of malaria.


Malaria is a disease caused by a protist called Plasmodium. Malaria kills just under half a million people a year worldwide.

Plasmodium is a parasite. It spends part of its life cycle in mosquitoes, and part in humans. The mosquito acts as a vector.

Mosquitoes that carry malaria are often found in areas with higher temperatures in Africa, Asia, and South and Central America.

In the life cycle of Plasmodium, a mosquito sucks blood containing the protists from an infected person. The mosquito passes the protist to other people when it sucks their blood. The mosquitoes are unaffected by the parasite, since they act as vectors.

The symptoms of malaria include a fever, sweats and chills, headaches, vomiting and diarrhoea.There is no vaccination for malaria. Infection can only be prevented by stopping individuals from being bitten by the mosquitoes. People sleep under mosquito nets and wear insect repellent to avoid bites. Antimalarial drugs are also taken, which treat the symptoms and can prevent infection.

Athlete's foot

Athlete's foot is a rash caused by a fungus that is found between people's toes. It causes the skin between the toes to become dry, itchy, red and cracked or scaly.

The fungus is often found in communal areas such as swimming pool or gym changing rooms. It is transmitted by walking on surfaces that have been previously contaminated or touching infected skin. Athlete's foot can be treated by antifungal medication.