Three types of parasites are commonly found in humans: ectoparasites, helminths and protozoa, according to the Centers for Disease Control and Prevention. These categories include parasites like fleas, ticks, one-celled organisms and various worms.
Ectoparasites include all blood-sucking parasites, such as mosquitoes, but the term is more commonly used to describe parasites that either burrow or attach to the skin, according to the CDC. These parasites include mites, ticks, fleas and lice. Ectoparasites, in addition to causing disease on their own, can transmit other pathogens that can be serious or fatal.
Helminths are worms that primarily live in the gastrointestinal tract, the CDC states. They include roundworms, thorny-headed worms and flatworms, which include tape worms and flukes. Adult helminths, which are large enough to be seen with the naked eye, cannot reproduce in humans. Roundworms can also live in the blood and lymph system, causing infection in tissues.
Microscopic protozoa are also common parasites in humans. Infectious protozoa types include sarcodina, ciliophora, mastigophora and sporozoa, according to the CDC. Protozoa can live in the bloodstream, spread by biting insects. They can also live in the intestinal tract, spread by ingested fecal matter from contaminated water or food. Protozoa multiply quickly and can cause a serious infection from just one organism.