- Protozoa - one-celled organisms that live and multiply in the blood or tissue of humans. They infect the body via mosquitoes and flies and are found in soil and water.
- Helminths - parasitic flatworms, flukes, tapeworms, thorny-headed worms, roundworms, and pinworms. They live in the gastrointestinal tract, blood, lymphatic system, and other tissues.
- Ectoparasites - ticks, fleas, lice, and mites that live on the surface of a human host and attach or burrow into the skin.
There are a host of parasitic infections that cause disease in humans. The effect can range from mildly annoying to life-threatening. Malaria is the most prevalent parasitic disease worldwide, killing more than 1 million people each year. Trichomoniasis, a common vaginal infection, is the most common parasitic infection in the U.S.