What infectious agent is present as protein capsules living within other body cells?

The correct answer is viruses. Viruses are unique infectious agents that consist of genetic material encased in a protein coat, known as a capsid. Unlike other infectious agents, viruses are not classified as living organisms because they cannot carry out metabolic processes outside a host cell.

When viruses infect a host, they penetrate the host cells and hijack the cellular machinery to replicate and produce new viral particles. This process often results in the destruction of the host cell and the release of new viruses that can go on to infect additional cells.

In contrast, bacteria are single-celled microorganisms that can live independently, fungi are complex organisms that can be unicellular or multicellular, and protozoa are also single-celled organisms that can survive and reproduce outside of host cells. These differences highlight why viruses specifically are described as protein capsules living within other body cells.