Protozoa are single-celled organisms that behave like tiny animals -- hunting and gathering other microbes for food. Protozoa can live within your body as a parasite. Many protozoa inhabit your intestinal tract and are harmless. Others cause disease, such as the 1993 Cryptosporidium parvum invasion of the Milwaukee water supply, sickening more than 400,000 people. Often, these organisms spend part of their life cycle outside of humans or other hosts, living in food, soil, water or insects.
Most protozoa are microscopic, but there are some exceptions. One type of ocean-dwelling protozoa (foraminifer) can grow to more than 2 inches in diameter.
Some protozoa invade your body through the food you eat or the water you drink. Others can be transmitted through sexual contact. Still others are vector-borne, meaning they rely on another organism to transmit them from person to person. Malaria is an example of a disease caused by a vector-borne protozoan parasite. Mosquitoes are the vector transmitting the deadly parasite plasmodium, which causes the disease.
Infection by one type of roundworm, known as a hookworm, can cause problems in your small intestine or lungs. The average hookworm is about half an inch long.