Parasitology is the study of parasites. These can be animal parasites (such as toxoplasma or heartworms) or human parasites. Parasitologists study multicellular parasites such as ascaris lumbricoides or flukes or unicellular eukaryotes such as malaria or leishmaniasis. In general, prokaryotes fall under the field of bacteriology rather than parasitology.
See also: Veterinary Parasitology