What are protozoa primarily known as?

Protozoa are primarily known as parasites because many species of protozoa are single-celled organisms that can live at the expense of other organisms, often causing disease. They can inhabit various environments and are capable of either free-living or parasitic lifestyles. Parasitic protozoa can infect humans and animals, resulting in significant health issues, such as malaria and amoebic dysentery.

While it is important to recognize that some protozoa are not parasitic and can exist independently in aquatic environments, the question focuses on their notable role in causing infections and diseases. This context emphasizes the significance of protozoa in parasitic relationships, which is why they are commonly referenced in terms of parasitism. Other options represent different classifications of living organisms that do not accurately describe protozoa. For example, bacteria are prokaryotic microorganisms, insects are a class of arthropods, and worms refer to various multicellular organisms, none of which encompass the defining characteristics of protozoa as effectively as the term "parasites."