Parasites can be found throughout the human body depending upon the parasite type. The three main classes of parasites that cause disease in humans include protozoa, ectoparasites and helminths.
A parasite gets its food at the expense of the host on which it is feeding. Protozoa are one-celled organisms that are able to multiply in human blood or tissue. Protozoa are classified into four groups: Sarcodina, Mastigophora, Ciliophora and Sporozoa.
Helminths are large organisms that are visible with the naked eye and include flatworms, thorny-headed worms and roundworms. The flatworms and thorny-headed worms are found in the gastrointestinal tract. Roundworms are found in the blood, gastrointestinal tract, lymphatic system or subcutaneous tissue. Ectoparasites are blood-sucking arthropods such as ticks, mites, mosquitoes and fleas.