Which of the following lists the three major types of parasitic diseases?

• A. Protozoa, Helminths, and Ectoparasites
• B. Protozoa
• C. Helminths
• D. Ectoparasites

Answer: (A)

Parasitic diseases are categorized by the type of parasite causing them: protozoa are single-celled organisms, helminths are multicellular worms (such as nematodes, cestodes, and trematodes), and ectoparasites live on the exterior of the host (like mites, fleas, lice, and ticks). This framework groups the main human infections into three broad categories, reflecting the biology and life cycles of the parasites involved. For example, malaria and amoebic dysentery illustrate protozoa; roundworm, whipworm, tapeworm, and fluke infections illustrate helminths; and scabies, pediculosis, and many tick- or flea-borne diseases illustrate ectoparasites. The option that lists all three—protozoa, helminths, and ectoparasites—best reflects these major categories, whereas the other choices mention only one group.