Parasites are a very diverse group of organisms that live off other living things, called hosts. Many parasites are harmless to their hosts, but some can cause serious diseases. The study of parasites is called parasitology.
There are three main types of parasites: protozoa, helminths, and arthropods. Protozoa are single-celled organisms that can only be seen with a microscope. Helminths are worms that can range in size from the very small (such as pinworms) to the very large (such as tapeworms). Arthropods are animals that have jointed legs and include insects (such as fleas and lice) and arachnids (such as mites and ticks).
Most parasites live in the digestive system of their host, where they can get access to food. Some parasitic worms, however, live in other parts of the body such as the lungs or liver. And some arthropods live on the outside of their host’s body.
Some parasites only infect one type of animal, while others can infect many different types of animals or even humans. For example, Giardia is a protozoan parasite that commonly infects dogs and cats but can also infect humans; whereas Trichinella spiralis is a worm parasite that primarily infects pigs but can also infect humans if they eat infected pork products.
Many people think of parasites as “germs” or “bugs” that cause diseases such as malaria or cholera. However, not all parasites cause disease; in fact, most do not. Some parasites may actually be beneficial to their hosts by helping them digest food or by killing harmful bacteria in their gut. It is only when a parasite grows too large or multiplies too rapidly inside its host that it becomes harmful and causes disease symptoms such as diarrhea, vomiting, weight loss, anemia or fever. In severe cases, parasitic infections can lead to death.
A few examples of parasitic diseases include:
Malaria – caused by Plasmodium protozoa which are transmitted by female Anopheles mosquitoes
Chagas disease – caused by Trypanosoma cruzi protozoa which are transmitted through contact with feces from triatomine bugs
Elephantiasis – caused by Wuchereria bancrofti worms which are transmitted by mosquito bites
River blindness – caused Onchocerca volvulus worms which are transmitted by blackfly bites