What type of organisms are protozoa classified with?

Protozoa are classified as parasites primarily because many species within this group exhibit parasitic behavior, living on or inside a host organism and deriving nutrients at the host's expense. They are single-celled eukaryotic organisms and can cause various diseases in humans and animals, demonstrating behaviors typical of parasites. Common examples include Giardia and Plasmodium, which are known to cause gastrointestinal and malaria infections, respectively.

While protozoa share some characteristics with other microorganisms, such as being microscopic and capable of causing disease, the term "parasite" specifically highlights their interaction with host organisms, setting them apart from bacteria, fungi, and viruses. Bacteria and fungi, for example, represent different domains of life with distinct characteristics and roles in the ecosystem, while viruses are essentially packets of genetic material that require a host cell to replicate and do not exhibit cellular structures like protozoa. Hence, the classification of protozoa as parasites emphasizes their role in infections and their reliance on other living organisms for survival.