What is a protozoan?

A protozoan is defined as a one-celled, simple organism belonging to the kingdom Protista. These microscopic entities are characterized by their single-celled structure, which allows them to perform essential life processes independently. Protozoans can be free-living or parasitic, and they exhibit various forms of movement, such as using flagella, cilia, or pseudopodia. This classification sets them apart from multicellular organisms and more complex life forms.

The other options describe entities that are significantly different from protozoans. Multicellular parasites consist of many cells and typically exhibit a higher level of complexity than single-celled organisms. Viruses, on the other hand, are not classified as living organisms and require a host to reproduce, which distinguishes them from protozoans. Lastly, bacterial infections are caused by bacteria, which are prokaryotic organisms, unlike the eukaryotic protozoans. Therefore, identifying protozoans accurately as simple, one-celled organisms is crucial for understanding microbiology and disease mechanisms.