Parasitism is the most common way of life; more than 50% of animal species are parasites. They occur in all animals. Many have a profound affect on the health of people, domestic animals, livestock and wild animals.
Parasites include fleas, lice, ticks, tapeworms, flukes, many nematodes and protozoa and some crustaceans.
Parasitology is the study of parasitism - the parasites themselves, how they infect hosts, diseases they cause, and immune responses to them.
Parasitologists study the life cycles, evolution, taxonomy, molecular biology, behaviour, ecology, pathology, epidemiology, treatment of infections and interactions between hosts and parasites.
Courtesy of University of Queensland website.