Parasites
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 are very tiny, microscopic, one-celled organisms that can live without a host or be parasitic in nature. They are able to reproduce in humans. Protozoa that live in the human intestine can be transmitted to another human most commonly by 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 can be transmitted to other humans most commonly through the bite of a mosquito or sand fly.
Helminths are large, multicellular organisms which can be seen with the naked eye when in their adult stages. They can be either free-living or parasitic in nature, and in their immature form can multiply in humans. The three main groups of helminths that are human parasites are; flatworms - these include flukes and tapeworms, thorny-headed worms, and roundworms - the adult forms of these worms can reside in the gastrointestinal tract, blood, lymphatic system or subcutaneous tissues. Alternatively, the immature (larval) states can cause disease of various body tissues because they can reproduce in the body. Some consider the helminths to also include leeches. However, these organisms are not typically considered parasites.
Ectoparasites can include mosquitoes, although this term is generally used more specifically to refer to ticks, fleas, lice, and mites that attach or burrow into the skin and remain there for relatively long periods of time (e.g., weeks to months).