August 13, 2022
A virus is a small piece of organic material that can only reproduce in cells of living things. When a virus invades a living cell, this cell – a so-called host cell – will start producing large quantities of copies of the original virus. Viruses infect all forms of life, from animals, fungi and plants to microorganisms such as bacteria and archaea. They occur in all ecosystems and are very numerous in the soil, air and water. The science that studies viruses is called virology, a subfield of microbiology. Viruses have a relatively simple structure. An individual virus particle is called a virion and contains the genetic material on the inside. This consists of a long DNA or RNA molecule that contains instructions for making proteins. A virus always has a protein coat, the capsid, that surrounds and protects the genetic material. In some cases, an outer sheath of lipids, called an envelope, is present. Most viruses are so small that they cannot be seen with a light microscope; they are about a hundred times smaller than most bacteria. Viruses spread in various ways, for example via exhaled aerosols or via vectors such as mosquitoes and ticks. In order to eliminate invading viruses, vertebrates have a complex immune system. Some viruses that are dangerous for humans, such as herpes viruses or HIV, can escape the immune system. Various vaccines have been developed to prevent viral infections. Successful vaccination strategies have now eradicated some notorious viral diseases. The evolutionary origin of viruses is unclear. They probably originated several times from 'escaped' pieces of DNA or RNA from living organisms. There is also no consensus on whether viruses themselves are alive. Although they possess genetic material and evolve, they do not have their own metabolism and they are unable to reproduce on their own. They are completely dependent on their host for their replication. Viruses have been described as "organisms on the edge of life" for this reason.