Viruses are small germs (pathogens) that can make people sick. They’re different from bacteria, plants, and animals because they can only infect specific types of cells. Viruses are made up of short sequences of genetic code wrapped in a protective protein shell called a capsid.
Most viruses are too small to see without a microscope. They can range in size from 20nm (nanometers) to 400nm. Viruses are too big to fit inside a living cell, so they have to infect a cell in order to reproduce themselves.
Like all living things, viruses contain nucleic acid—the genetic instructions that determine the structure and function of organisms. Unlike other organisms, however, viruses contain either RNA or DNA as their nucleic acid. In addition, while the DNA in most organisms is a double-stranded molecule, viruses can have either single-stranded RNA or double-stranded DNA.
Infection by a virus requires a viral receptor on the surface of a host cell. Some viruses have specific receptors that bind only to one tissue or organ, such as the hepatitis virus which attaches to cells on the lining of the liver. Other viruses, such as the common cold virus, bind to receptors on the lining of the nose.
Once a virus has bound to a host cell, it can start producing more viruses, which are released from the host cell and infect other cells. Viruses can spread between people in many ways, including touching surfaces or objects that an infected person with the virus has touched (like doorknobs or phones), having vaginal, oral, or anal sex, getting bit by an infected insect or animal, or drinking contaminated water. Almost all human viruses are contagious and can be passed from one person to another.
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