Which of the following are acellular infectious agents?

• A. Bacteria, archaea
• B. Viruses, viroids, and prions
• C. Fungi
• D. Protozoa

Answer: (B)

The concept here is recognizing what counts as an acellular infectious agent versus a cellular organism. Acellular infectious agents lack any cellular structure and cannot carry out metabolism on their own. Viruses fit this because they are essentially a genome enclosed in a protein shell and they must hijack a host cell’s machinery to replicate. Viroids are even simpler—small circular RNA molecules without a protein coat that rely on host enzymes to propagate, typically in plants. Prions are infectious proteins with no nucleic acid at all; they propagate by inducing normal proteins to misfold. None of these have cells or independent metabolic activity, so they are acellular.

By contrast, bacteria and archaea are cellular prokaryotes with membranes, cytoplasm, ribosomes, and their own metabolic processes. Fungi are eukaryotic organisms made of cells (and can be multicellular or unicellular like yeasts). Protozoa are single-celled eukaryotes with membrane-bound organelles. Since they all have cellular structure and metabolism, they are not acellular infectious agents.