Home > Encyclopedia of Science > Virus

• Words to Know
• Structure of viruses
• Viral infection
• Retroviruses
• The common cold and influenza

A virus is a small, infectious agent that is made up of a core of genetic material surrounded by a shell of protein. The genetic material (which is responsible for carrying forward hereditary traits from parent cells to offspring) may be either deoxyribonucleic acid (DNA) or ribonucleic acid (RNA). Viruses are at the borderline between living and nonliving matter. When they infect a host cell, they are able to carry on many life functions, such as metabolism and reproduction. But outside a host cell, they are as inactive as a grain of sand.

Viruses cause disease by infecting a host cell and taking over its biochemical functions. In order...