Question: how do parasites enter the human body?

Erin Pallott answered on 9 May 2024:

Parasites are a big group of pathogens! It includes the helminths (the worms), the protozoa (single-celled), and the ectoparasites which include things like lice and fleas and live on the surface of the body.

Parasites that invade the body can enter through a lot of different methods, but to list some common infection routes:
– Oral infection: This generally comes from eating food or drinking water that is contaminated with parasites or their eggs. Many worm infections like Trichuris and Ascaris shed eggs through faeces, and are passed on if someone swallows them. Undercooked meat is also a source of infection. Tapeworms and Toxoplasma can form cysts in animal muscle, which passes to humans if the meat is not fully cooked.
– Insect bites: Malaria is the disease caused by the Plasmodium parasite, which is passed through mosquito bites. Filarial worms are another example, passed through black fly bites.
– Skin penetration: Worm parasites including hookworm (on grass/plants) and schistosomes (in water) can directly penetrate the skin and travel in the bloodstream to the infection site.
– There are more ways someone can be infected with specific diseases such as sexual transmission, blood transfusions, and from mother to child, but the above are the more common routes.

Many parasite infections, especially intestinal worms that I work on, can be prevented by access to clean water and sanitation systems. Most people in the world are infected with at least one species of parasite, and they greatly range in how much harm they can do. Some are deadly, and some people wouldn’t ever know they’re infected.