What is a virus?

A virus is defined as an infectious agent that is indeed smaller than bacteria. Viruses are unique in that they cannot reproduce on their own and require a host cell to replicate. This distinguishes them from living organisms, as they do not exhibit characteristics of life outside of a host. They lack cellular structures and are composed of a core of genetic material, either DNA or RNA, encased in a protein coat. This microscopic size and reliance on host organisms for propagation contribute significantly to their classification as infectious agents.

In contrast, the other options describe different biological entities or incorrect classifications. A virus cannot be classified as a parasitic organism in the same way as some larger parasites that live independently and consume host resources without taking over the host cells. Also, they do not fit the description of skin cells or bacteria, as those are entirely different types of cells. These distinctions underscore the unique nature of viruses in the larger context of biology and infectious diseases.