Which of the following is a characteristic of viruses?

Viruses are unique entities in the realm of microbiology and have distinct characteristics that set them apart from other microorganisms. The correct choice highlights that viruses require a host cell to replicate. Unlike other living organisms, viruses cannot reproduce independently; they need to infect a host cell to hijack its biological machinery for their replication process. This dependency on a host cell is a fundamental characteristic of viruses, making them obligate intracellular parasites.

In contrast, other options do not accurately describe viruses. For instance, since they cannot reproduce on their own, the assertion that they can is incorrect. Additionally, viruses are generally much smaller than bacteria, which contradicts the notion that they are larger. Lastly, while some viruses can cause diseases, others may not have any harmful effects, meaning they are not always harmless. Thus, the dependence on a host cell for replication is the key characteristic that identifies viruses.