A virus is a tiny particle containing genetic material (DNA or RNA) enclosed in a protein shell. Viruses can infect cells of a wide variety of living organisms, including bacteria, plants and animals, but they cannot replicate outside the cell of a host organism. Viruses are so small that they can only be seen under an electron microscope. The study of viruses is known as virology.
Unlike the living, single-cell microorganisms such as bacteria, fungus or protozoa that make up most of our body’s cells, viruses do not have their own chemical machinery for performing the fundamental biological reactions that give rise to life. They need a host cell to survive and reproduce, so they attach to the outside surface of the host cell and then “hack” it into replicating itself with viral genetic instructions. The host cell, however, does not have the protein machinery to do this, so it either explodes or runs out of energy.
The virus particles, which are also called virions, are extremely small and can be passed on to others by direct contact or airborne droplets. The virus’s outer layer may contain spiky spikes that help it latch onto and enter the cells of other organisms. Viruses are so small that, like pollen or dust, they can pass through filters that normally retain larger particles such as bacteria and fungus.
Viruses can cause many different diseases, from the mild to the severe. Some are even deadly. Scientists are studying how viruses work and trying to find ways to prevent and cure them.