PLTW Medical Detectives Practice Exam

Bacteria are single-celled, independent organisms that can live and reproduce on their own in a variety of environments. They are prokaryotes, meaning their cells lack a nucleus, and they typically divide by binary fission, allowing them to thrive without a host.

Viruses, by contrast, aren’t cells and cannot replicate on their own; they must hijack a host cell to reproduce. Fungi include both multicellular forms and some single-celled yeasts, but many fungi are multicellular, and the term doesn’t specify a single-celled, free-living lifestyle. Microorganism is a broad label that covers many tiny life forms, including both free-living bacteria and parasitic viruses, so it’s not the precise description asked for here.

So, the single-celled organisms that do not rely on a host are bacteria.