Medical parasitology has traditionally involved the study of three main groups of animals: parasitic protozoa, parasitic helminths (worms), and arthropods that directly cause disease or act as carriers of various pathogens. A parasite is a pathogen that simultaneously injures the host and obtains food. Some organisms, called parasites, are actually commensals because they neither benefit nor harm their hosts.
Human infections caused by parasites number in the billions and range from relatively harmless to fatal. Diseases caused by these parasites are the main problems of human health all over the world.
During their life, parasitic organisms usually go through several developmental stages, during which there are changes not only in structure, but also in biochemical and antigenic composition.