Which group consists of one-celled organisms capable of multiplying in humans and may be free-living or parasitic?

• A. Protozoa
• B. Bacteria
• C. Viruses
• D. Fungi

Answer: (A)

Protozoa are microscopic, single-celled organisms that can multiply inside humans. They can live freely in the environment or as parasites, and many species complete their life cycles by producing forms that proliferate within the host. This combination of unicellularity, ability to replicate in the human body, and the option to be free-living or parasitic is characteristic of protozoa, which include agents like Plasmodium, Giardia, and Entamoeba. In contrast, viruses are not cells and require host machinery to replicate, bacteria are prokaryotic and differ in cellular structure, and fungi are typically multicellular (with some yeast-like unicellular forms) rather than a standard description of a single-celled parasite that multiplies within humans.