What is a key characteristic of viruses?

A key characteristic of viruses is that they require a host to replicate. Unlike bacteria, which can reproduce independently in suitable environments, viruses depend on the cellular machinery of a host organism to multiply. Once a virus enters a host cell, it hijacks the cell's processes to create copies of itself, ultimately leading to the release of new viral particles that can infect additional cells. This dependency on a host for reproduction is a fundamental aspect of what defines a virus as a biological entity.

The other answer choices misunderstand important aspects of viral biology. For example, viruses are generally much smaller than bacteria, making the claim that they are larger incorrect. Also, not all viruses induce diseases; while many do, there are also viruses that can exist in a host without causing illness. Therefore, the requirement of a host for replication stands as a defining characteristic of viruses.