A virus is a biological agent that reproduces inside the cells of living hosts. When infected by a virus, a host cell is forced to produce many thousands of identical copies of the original virus, at an extraordinary rate. Unlike most living things, viruses do not have cells that divide; new viruses are assembled in the infected host cell. Over 2,000 species of viruses have been discovered, making them members of one of the smaller kingdoms of organisms.
A virus consists of two or three parts: Genes, made from either DNA or RNA, long molecules that carry the genetic information; a protein coat that protects the genes; and some viruses have an envelope of fat that surrounds and protects them when they are not contained within a host cell. Viruses vary in shape from the simple helical and icosahedral to more complex structures. Viruses are about 100 times smaller than bacteria, it would take 30,000 to 750,000 of them, side by side, to stretch to 1 centimetre (0.39 in).
Viruses spread in many different ways. Plant viruses are often spread from plant to plant by insects and other organisms, known as vectors. Some viruses of animals are spread by blood-sucking insects. Each species of virus relies on a particular method. Whereas viruses such as influenza are spread through the air by people when they cough or sneeze, others such as norovirus, which are transmitted by the faecal-oral route, contaminate hands, food and water. Rotavirus is often spread by direct contact with infected children. HIV, is one of several major viruses that are transmitted during sex. The origins of viruses is unclear: some may have evolved from plasmids—pieces of DNA that can move between cells—while others may have evolved from bacteria.
Viral infections often cause disease in humans and animals, however they are usually eliminated by the immune system, conferring lifetime immunity to the host for that virus. Antibiotics have no effect on viruses, but antiviral drugs have been developed to treat life-threatening infections. Vaccines that produce lifelong immunity can prevent some viral infections.
In 1884, the French microbiologist Charles Chamberland invented a filter, (known today as the Chamberland filter or Chamberland-Pasteur filter), that has pores smaller than bacteria. Thus, he could pass a solution containing bacteria through the filter and completely remove them from the solution. Russian biologist Dimitri Ivanovski used this filter to study what is now known to be the tobacco mosaic virus. His experiments showed that the crushed leaf extracts of infected tobacco plants are still infectious after filtration.
At the same time several other scientists proved that, although these agents (later called viruses) were different from bacteria, they could still cause disease, and they were about a hundred times smaller than bacteria. In 1899 The Dutch microbiologist Martinus Beijerinck observed that the agent multiplied only in dividing cells. Having failed to demonstrate its particulate nature he called it a "contagium vivum fluidum" to mean "soluble living germ". In the early 20th century, English bacteriologist Frederick Twort discovered viruses that infect bacteria, and French-Canadian microbiologist Félix d'Herelle described viruses that, when added to bacteria growing on agar, would lead to the formation of whole areas of dead bacteria. Counting these dead areas allowed him to calculate the number of viruses in the suspension.
With the invention of electron microscopy in 1931 by the German engineers Ernst Ruska and Max Knoll came the first images of viruses. In 1935 American biochemist and virologist Wendell Meredith Stanley examined the tobacco mosaic virus and found it to be mostly made from protein. A short time later, this virus was separated into protein and RNA parts. A problem for early scientists was that they did not know how to grow viruses without using live animals. The breakthrough came in 1931, when the American pathologist Ernest William Goodpasture grew influenza and several other viruses in fertilised chickens' eggs. Some viruses could not be grown in chickens' eggs, but this problem was solved in 1949 when John Franklin Enders, Thomas Huckle Weller and Frederick Chapman Robbins grew polio virus in cultures of living animal cells. Over 2,000 species of virus have been discovered.
Viruses are found wherever there is life and have probably existed since living cells first evolved. The origin of viruses is unclear because they do not form fossils, so molecular techniques have been the most useful means of hypothesising how they arose. However, these techniques rely on the availability of ancient viral DNA or RNA but most of the viruses that have been preserved and stored in laboratories are less than 90 years old. Molecular methods have only been successful in tracing the ancestry of viruses that evolved in the 20th century.
There are three main theories of the origins of viruses:
- Coevolution theory: Viruses may have evolved from complex molecules of protein and DNA at the same time as cells first appeared on earth and would have been dependent on cellular life for many millions of years.
- Cellular origin theory : Some viruses may have evolved from bits of DNA or RNA that "escaped" from the genes of a larger organism. The escaped DNA could have come from plasmids—pieces of DNA that can move between cells—while others may have evolved from bacteria.
- Regressive theory: Viruses may have once been small cells that parasitised larger cells. Over time, genes not required by their parasitism were lost. The bacteria rickettsia and chlamydia are living cells that, like viruses, can reproduce only inside host cells. They lend credence to this theory, as their dependence on parasitism is likely to have caused the loss of genes that enabled them to survive outside a cell.
A virus particle, known as a virion, consists of genes made from DNA or RNA which are surrounded by a protective coat of protein called a capsid. The capsid is made of many smaller, identical protein molecules which are called capsomers. The arrangement of the capsomers can either be icosahedral (20-sided), helical or more complex. There is an inner shell around the DNA or RNA called the neucleocapsid, which is formed by proteins. Some viruses are also surrounded by a bubble of lipid (fat) called an envelope envelope. Viruses are among the smallest infectious agents, and most of them can only be seen by electron microscopy. Most viruses cannot be seen by light microscopy (in other words, they are sub-microscopic); their sizes range from 20 to 300 nm They are so small that it would take 30,000 to 750,000 of them, side by side, to stretch to one cm.
Genes are made from DNA (deoxyribonucleic acid) and, in many viruses, RNA (ribonucleic acid). The biological information contained in an organism is encoded in its DNA or RNA. Most organisms use DNA, but many viruses have RNA as their genetic material. The DNA or RNA of viruses consists of either a single strand or a double helix.
Viruses reproduce rapidly because they have only a few genes compared to humans who have 20,000–25,000. For example, influenza virus has only eight genes and rotavirus has eleven. These genes encode structural proteins that form the virus particle, or non-structural proteins, that are only found in cells infected by the virus.
All cells, and many viruses, produce proteins that are enzymes called DNA polymerase and RNA polymerase which make new copies of DNA and RNA. A virus's polymerase enzymes are often much more efficient at making DNA and RNA than the host cell's. However, RNA polymerase enzymes often make mistakes, and this is one of the reasons why RNA viruses often mutate to form new strains. In some species of RNA virus, the genes are not on a continuous molecule of RNA, but are separated. The influenza virus, for example, has eight separate genes made of RNA. When two different strains of influenza virus infect the same cell, these genes can mix and produce new strains of the virus in a process called reassortment.