AEMCA Practice Exam 2026 – 400 Free Practice Questions to Pass the Exam

A virus is defined biologically as a microscopic infectious agent that contains either DNA or RNA as its core genetic material and is incapable of independent reproduction. Instead, viruses must invade host cells in order to replicate and propagate. In doing so, they hijack the cellular machinery of the host, leading to the production of new virus particles. This dependency on host cells for replication is a key characteristic that sets viruses apart from living organisms, which typically can reproduce on their own.

The other options depict characteristics of different types of organisms or biological entities. For instance, single-celled organisms that can reproduce independently refer to bacteria or protists, which can live and multiply on their own without the need for a host. Meanwhile, multicellular organisms that do not cause infection might include plants or animals, and fungi that break down organic matter are critical to nutrient cycling in ecosystems, but none fulfill the criteria that define a virus. Thus, the accurate definition of a virus centers around its structure and reproductive strategy involving host cells.