Parasites

**About Parasites** Key points - A parasite is an organism that lives on or in a host organism and gets its food from or at the expense of its host. - There are three main classes of parasites that can cause disease in humans: protozoa, helminths, and ectoparasites. Protozoa Protozoa are microscopic, one-celled organisms that can be free-living or parasitic in nature. They are able to multiply in humans, which contributes to their survival and also permits serious infections to develop from just a single organism. Transmission of protozoa that live in a human's intestine to another human typically occurs through a fecal-oral route (for example, contaminated food or water or person-to-person contact). Protozoa that live in the blood or tissue of humans are transmitted to other humans by an arthropod vector (for example, through the bite of a mosquito or sand fly). The protozoa that are infectious to humans can be classified into four groups based on their mode of movement: - Sarcodina – the ameba, e.g., Entamoeba - Mastigophora – the flagellates, e.g., [Giardia](https://www.cdc.gov/giardia/about/index.html), [Leishmania](https://www.cdc.gov/leishmaniasis/about/index.html) - Ciliophora – the ciliates, e.g., [Balantidium](https://www.cdc.gov/balantidium/about/index.html) - Sporozoa – organisms whose adult stage is not motile e.g., [Plasmodium](https://www.cdc.gov/malaria/index.html), [Cryptosporidium](https://www.cdc.gov/cryptosporidium/about/index.html)