Parasites comprise a distinct class of unicellular (protozoa) or multicellular (helminths) species that invade and cause disease in the host organism. Although parasitic infections can occur worldwide, the concentration of cases are distributed around the tropical or equatorial regions. There is also a greater incidence of disease among underdeveloped countries where hygiene and sanitation is a problem. Not commonly known in developed countries, high-profile parasites such as the malaria-causing Plasmodium falciparum infect and kill far more people every year than AIDS or cancer.