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How do protozoans, worms, and ectoparasites cause parasitic infections?

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Diagram showing the West Nile virus transmission cycle: a mosquito bites an infected bird (the primary reservoir host), then transmits the virus to humans or horses (incidental hosts) through subsequent bites. — infectious disease Birds are the primary reservoirs of West Nile virus. The virus is transmitted to humans and horses by mosquitoes that have bitten infected birds.

Parasitic infections are caused by organisms that live in or on a host, typically at the host's expense. These infections are primarily transmitted through the ingestion of contaminated food or water or via the bite of an arthropod vector.1

Protozoans

Protozoans cause disease by invading host cells or tissues. For example, Plasmodium parasites infect and replicate within liver and red blood cells, causing them to rupture. Others, such as Leishmania and Trypanosoma cruzi, are intracellular parasites that infect macrophages or muscle tissue. Some protozoans, like Trypanosoma brucei, evade the immune system through antigenic variation, changing their surface proteins to survive.23

Worms (Helminths)

Helminths, including nematodes and flatworms, cause disease through physical obstruction, tissue damage, or nutrient depletion. Tapeworms attach to the intestinal wall or burrow into organs like the brain, while hookworms cause anemia by feeding on blood.24

Ectoparasites

Ectoparasites, such as fleas, lice, and ticks, cause irritation or disease by attaching to the skin or burrowing into it. They also act as vectors, transmitting other pathogens like the bacteria responsible for Lyme disease.2