Human parasites are organisms that have evolved to derive some benefit from a human host at that host's expense. They may be relatively simple microorganisms or complex animals. Examples of human parasites include protozoa, helminths and ectoparasites, according to the Centers for Disease Control and Prevention.
Protozoan parasites are transmitted directly from person to person, or they may be present in contaminated food or water, according to the CDC. Giardia is a protozoan parasite, as are cryptosporidium and the malaria plasmodium. Helminths are worms that typically take advantage of human hosts during their juvenile or larval stages. Flukes and tapeworms are flatworms common to humans, and the intestinal parasite Ascaris ascarii is a type of roundworm. Though they may be very small, helminths are all multicellular organisms.
Ectoparasites are complex animals. Common human ectoparasites are ticks, mites and lice, reports the CDC. These ectoparasites are all arthropods that either attach themselves to the skin or hair, or they burrow into the skin and remain in place for extended periods. Scabies is an example of a parasitic mite, as are chiggers, both of which cause significant itching and discomfort for sufferers due to the secretion of digestive enzymes under the human host's skin.