Lice are parasites, but they don’t make you sick. They’re just annoying. (Istockphoto)

If I ask kids what makes them sick, they usually come up with such things as colds, the stomach flu or strep throat. Although viruses and bacteria cause most infections, there is another group of creatures that can put you on the sidelines: parasites.

A parasite is an organism that lives by “stealing” nutrients from another organism (the host). Parasites are divided into two categories. Ectoparasites live outside the host’s body. Endoparasites live inside the host’s body. There are thousands of species of parasites on Earth. They attack humans, plants and animals. Many animals harbor more than one type of parasite at the same time.

Parasites can be microscopic or macroscopic. The first type can be seen only with a microscope. One example is malaria. The second type can be seen with the “naked eye,” even though they are usually pretty small. Fleas fall into this category.

Some parasites have a simple life cycle. They hatch. They find a host to feed on. They reproduce. They die. Examples include lice and pinworms.

Lice attach to hairs near the scalp and merrily feed on the blood of their host. When the eggs hatch, the larvae (baby lice) need to find a host. Sometimes it’s the same head; other times it’s a new one. In many cases, the new host will be your brother or sister.

Pinworms live inside the large intestine, feeding on nutrients in your poop. Adult pinworms venture outside the host at night and deposit thousands of eggs around the anus, or bottom. (When kids get pinworms, they commonly wake up at night complaining of an itchy bottom. Now you know why.) Pinworm eggs spread on articles of clothing and fingers that weren’t washed well. The eggs are swallowed, mature into adult pinworms and begin the process again.

Some parasites have a complicated life cycle. They hatch. They spend part of their life in one host developing into a juvenile form. They move to a different host and mature into adulthood. Examples are the parasites that cause schistosomiasis (pronounced shis-te-so-MY-a-sis), which is found in tropical countries throughout the world. The eggs from infected people hatch in water and spend the first part of their life in snails. The juvenile forms return to the water and enter the human host through the skin. The parasites move to the liver, where they mature and reproduce.

Some parasites are a nuisance but don’t usually harm the host. Lice fit into this category. Although lice are disgusting and a pain to get rid of, they don’t spread disease. As a result, you could have lice for years and not suffer. You might not get invited to many sleepovers, but other than that your health would be okay.

Other parasites, such as giardia, make you sick. Giardia is usually picked up in freshwater rivers and lakes. It lives in the upper part of the small intestine. Symptoms include stomachache, nausea, vomiting, diarrhea, weight loss, anemia (low iron) or intestinal problems that don’t easily go away.

Despite its nasty reputation, giardia is the cutest parasite on the planet. Check out the image in the middle!

— Howard J. Bennett

Bennett is a Washington pediatrician. His Web site,, includes past KidsPost articles and other cool stuff.