Viruses are submicroscopic infectious agents that replicate only inside living cells. They infect all life forms, from animals and plants to microorganisms like bacteria and archaea. They are found in every ecosystem on Earth and are the most numerous type of biological entity.
Unlike bacteria, which can reproduce themselves independently of the host cell they inhabit, viruses cannot survive and must rely on the host for all their essential metabolic processes. Nevertheless, many viruses infect healthy hosts and can cause diseases like cancer or the new coronavirus that emerged in 2020 and caused the pandemic of COVID-19.
The structure of a single virus is a protein capsid that protects the core of genetic information, either DNA or RNA. Depending on the kind of virus, the capsid can have one of four different shapes: icosahedral, polyhedral, helical, or spherical.
A lipid membrane (sometimes called an envelope) can also surround some viral species, protecting the capsid from detergents and solvents. This envelope can have additional protein components arranged in a layer referred to as a tegument.
Viruses can be positive-sense, with a single set of instructions that assembles itself, or double-sense, with two sets of instructions that are paired together and then separated. Most RNA viruses are single-stranded, but some can have both positive- and negative-sense RNA.
Viruses must infect cells to make copies of themselves, so they have evolved proteins that can recognize and attach to cellular receptors on the surface of a host cell. Once the virus is attached, it releases its RNA or DNA into the cell and the cell’s machinery begins making more virus particles. The resulting virions can then infect other cells, and the process repeats itself.