A pathogen is a microorganism (e.g. virus, bacterium, protist, fungi) that causes disease in animals or plants. Disease can be caused by damaging cells of the infected organism by replicating inside them, or making toxins (poisons). This makes us feel ill if we are infected by a pathogen. The immune system protects us from infection by pathogens. Pathogens can be transmitted between hosts by direct contact, by water, air or contaminated surfaces.
Here are some examples of different types of pathogens, the diseases they cause and how they are transmitted.
Hepatitis C Virus infects and replicates in the human liver. By damaging liver cells this can cause liver disease and cancer. The virus can be transmitted between people by blood-to-blood contact; for example by sharing needles in IV drug use, or from mother to baby during childbirth.
Salmonella bacteria and the toxins they produce can cause food poisoning. Symptoms of food poisoning can include abdominal cramps, vomiting and diarrhoea. Salmonella are spread in ingested food or by food stored or prepared in unhygienic conditions.
Malaria is caused by protists called Plasmodium. These complicated pathogens have a life-cycle with different stages that allow it to live in two different hosts; humans and mosquitos. Humans become ill, but the mosquitos do not. The pathogen is spread when mosquitoes bite an infected person, take up some of the plasmodium and then bite an uninfected person. The mosquito is a ‘vector’ for the pathogen. Spread of the disease can be reduced by using mosquito nets, insecticides or by preventing mosquitos from breeding (To explore a new strategy, look up ‘self-limiting mosquitoes’). The pathogen replicates inside and destroys human blood cells, causing fevers and anaemia. They can also stick to the sides of blood vessels causing blockages that can affect the brain. Malaria still causes 400,000 deaths a year globally.
Additional information: A few pathogens are not microorganisms, but worms. For example, Tapeworms are parasites that live in the human gut and grow to several metres in length. Tapeworm eggs are excreted in human stools and if allowed to contaminate food can be transmitted to other people.