what is a virus definition

A virus is an infectious microbe consisting of a segment of nucleic acid (either DNA or RNA) surrounded by a protein coat. It is a small piece of genetic information (DNA or RNA) inside of a protective shell (capsid) that cannot replicate alone; instead, it must infect cells and use components of the host cell to make copies of itself. Viruses are so small that a microscope is necessary to visualize them, and they have a very simple structure. They can infect a variety of living organisms, including bacteria, plants, and animals. Viruses are quintessential parasites; they depend on the host cell for almost all of their life-sustaining. Some common diseases caused by viruses include the flu, the common cold, COVID-19, AIDS, measles, smallpox, chickenpox, and rabies.