Human parasites live in or on our bodies and take their nourishment from us. By definition, parasites cannot live on their own--they use us as their hosts. A variety of insects, worms and single-celled organisms cause parasitic infections, which number in the millions annually in the U.S.

Parasitic nematodes (worms) in humans can be found in the intestines, muscles and other tissues. More people around the world have a nematode infection than any other parasitic infection.

The human body can be a host to parasites, both internal and external. Some human parasites live in hair, while some simply find hair to be a convenient hiding spot. Understanding the symptoms of some of these common parasites can help you and your physician to properly diagnose and treat infestations.

As family members, dogs can transmit diseases to their human cohabitants. One class of disease that can be especially difficult to control due to the natural behavior of dogs is parasites. Dogs have no aversion to digging in the ground "bare pawed" or inspecting their own or others fecal deposits.

Parasite invasion usually occurs from the consumption of contaminated food or liquid. Bites from pests such as tics and mosquitoes is another common origin. Infection is common in young children, who tend to interact with parasite-infested environments such as gardens and playgrounds.