A virus is an infectious agent that can only replicate within a host organism. Viruses can infect a variety of living organisms, including bacteria, plants, and animals. Viruses are so small that a microscope is necessary to visualize them, and they have a very simple structure. When a virus particle is independent from its host, it consists of a viral genome, or genetic material, contained within a protein shell called a capsid. In some viruses, the protein shell is enclosed in a membrane called an envelope.
