There are hundreds of human body parasites, but they are usually grouped within three classes — protozoa, worms, and insect parasites. Protozoan organisms are one-celled parasites that often act like germs or viruses in the body to create disease-like symptoms. Worms almost always live within the body, feeding off blood, muscle and other organ tissues. Insects, lice and other bug-like parasites generally live in the hair, on the skin or just under the surface of the skin and often survive by consuming skin cells or blood.
Most protozoa are invisible to the naked eye, and it is often difficult to distinguish protozoan colonization from bacterial and viral infections. Many protozoan infections can be attributed to insect bites or food or water being contaminated by feces. Undercooked or raw meat, non-pasteurized milk and animal feces can also carry the parasites.
Malaria was once thought to be a viral infection but is now known to be caused by protozoan body parasites. The protozoa that cause the disease are transferred through mosquito bites. Toxoplasmosis is a protozoan disease present in cat feces and can cause birth defects in children of infected mothers. For this reason, pregnant women are advised against changing litter boxes. Sleeping sickness and amoebic dysentery, also known as Montezuma’s revenge, are also caused by protozoan parasites.
Worms are multi-celled body parasites that generally live inside humans and animals. Parasitic worms are generally classified as roundworms, tapeworms, or flukes, which are also known as flatworms. Although any body part can host worms, the stomach and the digestive tract in humans are the mostly likely areas to be infested.
Beef and pork tapeworms both infect the digestive tract and are usually transmitted by eating raw or undercooked meat. In rare cases, the beef tapeworm can grow up to 60 feet (18.28 meters). Hookworms are round worms that live in warm soil and are usually transmitted through the soles of bare feet. The intestinal fluke, Fasciolopsis buski, is one of the largest of the fluke classification of parasitic worms. They can grow to be around 3 inches (7.62 cm) long.
Any insects that feed on blood can technically be considered body parasites, but lice are considered particularly troublesome. These insects tend to colonize their host, feeding repeatedly. Head, body and pubic lice live on the skin and hair of humans and are spread easily through close contact with other infected people. These lice feed and lay eggs in the hair or on the clothing of infested humans.
Several types of insects have been known to lay eggs in human flesh. Once the eggs hatch, the larvae become parasitic, feeding from their human host until they reach adulthood. Botfly and blowfly larvae infestations, as well as common house fly maggot infestations, can occur and are a particular danger to people with open wounds.
Completely avoiding body parasites is difficult, but some precautions can reduce exposure. Meat and seafood should always be thoroughly cooked before eating. Care should be taken to cover exposed skin in areas with large insect populations, and shoes should always be worn when walking outside. Finally, whenever treated water is unavailable, drinking water should be brought to a full boil for at least five minutes before consuming.