Which type of infectious agent is present as protein capsules living within other body cells?

The correct answer is that viruses are the infectious agents that exist as protein capsules living within other body cells. Viruses are unique among pathogens because they require a living host cell to reproduce and carry out their life cycle. They invade host cells, where they hijack the cellular machinery to replicate their genetic material and produce new viral particles.

Viruses consist of a core of genetic material (either DNA or RNA) surrounded by a protein coat called a capsid. Some viruses may also have an outer envelope derived from the host cell membrane. This structure allows them to attach to and enter host cells and is essential for their ability to infect.

In contrast, bacteria are single-celled organisms that do not require host cells to replicate. Fungi are a separate category of organisms that can be single-celled or multicellular and also reproduce independently. Prions are misfolded proteins that can cause disease but do not have a traditional structure of viral capsids or require cellular hosts in the same way viruses do. Each of these other infectious agents has different characteristics and modes of action compared to viruses.