virus (biology)


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.


The principles of virology go back to the late 19th century. In 1883, Adolf Mayer, a German agronomist, discovered that he could transmit disease from his tobacco plants by rubbing the sap of diseased leaves on healthy plants. He failed to find the infectious pathogen, suggesting that the disease was caused by very small bacteria that were invisible under the microscope. This hypothesis was tested a decade later by Dmitri Ivanovski, a Russian biologist who passed the sap from infected tobacco plants through a special filter designed to trap bacteria. Even after filtration, the juice was still infectious to the tobacco plants. In 1898, the Dutch microbiologist Martinus Beijerinck repeated the experiments and came to the conclusion that the filtrate must contain a new type of pathogen. He found that the pathogen could only multiply in living cells. In later experiments, Beijerinck showed that the cause of the tobacco plant disease could not be grown on a nutrient medium. He hypothesized that it was a replicating particle much smaller and simpler than a bacterium, and coined the term virus. Beijerinck is considered one of the most important founders of virology. Beijerinck's suspicions were confirmed in 1935 when American Wendell Stanley crystallized the infectious particle, known today as the tobacco mosaic virus, and further described its properties. Not long after, the electron microscope was invented and viruses could be made visible for the first time.