Parasites are organisms which live in or on an organism of another species. There are three classes of parasites that can cause illness in humans. These are ectoparasites, helminths, and protozoa.
Protozoa and helminths usually affect the gut whereas the ectoparasites like mites and lice attach to or burrow into the skin and can remain there for long periods.
Most helminths and protozoa are non-pathogenic, i.e., they don’t cause illnesses or cause very mild illnesses. But, others can lead to serious diseases in humans.
