What is a virus?

A virus is a small infectious agent that can only replicate inside the cells of another organism. Viruses infect nearly all living organisms such as animals, plants and even single celled bacteria. Each type of virus is unique to the type of cell it infects, which means that the virus must find the correct host cell to replicate.


Viruses were first discovered in the 1898 by the Dutch scientist, Martinus Beijerinck. Beijerinck used a filtration experiment to show that a disease that infected tobacco plants was caused by something smaller than bacteria, which he called a virus. The virus he discovered is the tobacco mosaic virus. Today there are millions of different types of viruses, of those about 5,000 are known in detail.

What do they look like?

Viruses had been undetected throughout most of history because they are incredibly small. Most are so small they must be measured in nanometers (approximately a billionth of a meter). Viruses were first visualized the 1930’s with the invention of the electron microscope.

Viruses come in a variety of different shapes and sizes, and can look as simple as a rod or be very complex with several different parts. Below, viral shapes from left to right are: helical, icosahedral, envelope, and complex.

Viruses: helical, icosahedral, envelope, and complex

All viruses consist of two or three parts: genes made from DNA or RNA, a protein coat to protect those genes, and some have a phospholipid envelope that surrounds them when they are outside of a cell.

What do they do?

Viruses replicate by invading specific host cells and then using those cells to create more viruses. This process often causes the cells to die, causing disease. The diseases viruses cause range in severity from the common cold to AIDS. Some other well known diseases caused by viruses are the fl u, hepatitis, chicken pox, and cold sores.

How do they spread?

Viruses use several different routes of transmission to move from one organism to another. Some can be passed on through the air we breathe, others need to be ingested or transferred through blood or bodily fluids. Some can even be transmitted just by touching someone else. That is why washing your hands before you eat, staying away from others when you feel sick, and avoiding someone else’s bodily fluids is so important.

Fun Fact:

Did you know that viruses are the most common biological entity in the ocean? There are millions of virus particles floating in each cubic millimeter (a small drop) of ocean water. If you took all the viruses in the ocean and lined them up end to end, they would reach 200 light years into space.