A parasite is a microorganism that lives within and gets its food from a host. Some of these relationships are antagonistic, where parasites cause disease in the host. Parasitic infections are more common in tropic and subtropic regions as well as more temperate climates.
The three main classes of parasites that cause disease in humans are protozoa, helminths, and ectoparasites. Of all the parasitic diseases, malaria causes the greatest number of deaths globally (approximately 660,000 people/year). There are several other parasitic diseases that have been labelled as "Neglected Tropical Diseases" due to the lack of attention from the public health community. These diseases include lymphatic filariasis, onchocerciasis, and Guinea worm disease.
Check out the video above which shows parasitic microorganisms in action under the microscope. The organisms in the video were collected from Australian sharks and stingrays and visualized under the microscope by Dr. Leslie Chisholm at the University of Adelaide in Australia.
Sources: CDC, Dr. Leslie Chisholm