Four major groups of parasites are Protozoa, Nematodes, Cestodes, and Trematodes. Through a transmitting agent, such as a mosquito. Through sexual conduct. Via the nose and skin. The parasite is often harmful.
Most parasites produce similar symptoms, as noted above.
Once in your body, different parasites behave differently. Parasites can attach themselves to the body and suck out its nutrition. While some parasites stay in the intestinal tract, others explore the body, passing through any number of body parts, and often causing havoc as they go. They can, for example, get into joints and eat the calcium linings of your bones, resulting in arthritic tendencies. They can also eat the protein coating on your nerves, causing a disruption in the nerve signals from the brain.