The adjective viral dates to ,  the term virion plural virions , which dates from ,  is also used to refer to a single, stable infective viral particle that is released from the cell and is fully capable of infecting other cells of the same type.
History of virology and Social history of viruses Louis Pasteur was unable to find a causative agent for rabies and speculated about a pathogen too small to be detected using a microscope. Thus, he could pass a solution containing bacteria through the filter and completely remove them;  in , the Russian biologist Dmitri Ivanovsky used this filter to study what is now known as the tobacco mosaic virus. His experiments showed that crushed leaf extracts from infected tobacco plants remain infectious after filtration.
He accurately diluted a suspension of these viruses and discovered that the highest dilutions lowest virus concentrations , rather than killing all the bacteria, formed discrete areas of dead organisms. Counting these areas and multiplying by the dilution factor allowed him to calculate the number of viruses in the original suspension. By the end of the 19th century, viruses were defined in terms of their infectivity , their ability to be filtered, and their requirement for living hosts.
Viruses had been grown only in plants and animals; in , Ross Granville Harrison invented a method for growing tissue in lymph , and, in , E. Lambert used this method to grow vaccinia virus in fragments of guinea pig corneal tissue;  in , H. Maitland grew vaccinia virus in suspensions of minced hens' kidneys, their method was not widely adopted until the s, when poliovirus was grown on a large scale for vaccine production.
This work enabled Jonas Salk to make an effective polio vaccine. The first X-ray diffraction pictures of the crystallised virus were obtained by Bernal and Fankuchen in , on the basis of her pictures, Rosalind Franklin discovered the full structure of the virus in In , the hepatitis B virus was discovered by Baruch Blumberg ,  and in , Howard Temin described the first retrovirus.
Viral evolution 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 used to compare the DNA or RNA of viruses and are a useful means of investigating how they arose. There are three main hypotheses that aim to explain the origins of viruses: 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 support to this hypothesis, as their dependence on parasitism is likely to have caused the loss of genes that enabled them to survive outside a cell, this is also called the degeneracy hypothesis,   or reduction hypothesis.
The escaped DNA could have come from plasmids pieces of naked DNA that can move between cells or transposons molecules of DNA that replicate and move around to different positions within the genes of the cell. Viroids are molecules of RNA that are not classified as viruses because they lack a protein coat. They have characteristics that are common to several viruses and are often called subviral agents.
Although hepatitis delta virus genome may replicate independently once inside a host cell, it requires the help of hepatitis B virus to provide a protein coat so that it can be transmitted to new cells;  in similar manner, the sputnik virophage is dependent on mimivirus , which infects the protozoan Acanthamoeba castellanii.
The virus-first hypothesis contravened the definition of viruses in that they require host cells. To date, such analyses have not proved which of these hypotheses is correct,  it seems unlikely that all currently known viruses have a common ancestor, and viruses have probably arisen numerous times in the past by one or more mechanisms.
Although they have genes, they do not have a cellular structure, which is often seen as the basic unit of life. Virus self-assembly within host cells has implications for the study of the origin of life , as it lends further credence to the hypothesis that life could have started as self-assembling organic molecules. RNA coiled in a helix of repeating protein sub-units Structure of icosahedral adenovirus. Electron micrograph of with a cartoon to show shape Structure of chickenpox virus.
They have a lipid envelope Structure of an icosahedral cowpea mosaic virus Viruses display a wide diversity of shapes and sizes, called morphologies ; in general, viruses are much smaller than bacteria. When virions are coated with stain positive staining , fine detail is obscured. Negative staining overcomes this problem by staining the background only. Complex viruses code for proteins that assist in the construction of their capsid. Proteins associated with nucleic acid are known as nucleoproteins , and the association of viral capsid proteins with viral nucleic acid is called a nucleocapsid, the capsid and entire virus structure can be mechanically physically probed through atomic force microscopy.
Helical These viruses are composed of a single type of capsomere stacked around a central axis to form a helical structure, which may have a central cavity, or tube.
This arrangement results in rod-shaped or filamentous virions: These can be short and highly rigid, or long and very flexible, the genetic material, in general, single-stranded RNA, but ssDNA in some cases, is bound into the protein helix by interactions between the negatively charged nucleic acid and positive charges on the protein. Overall, the length of a helical capsid is related to the length of the nucleic acid contained within it and the diameter is dependent on the size and arrangement of capsomeres, the well-studied tobacco mosaic virus is an example of a helical virus.
A regular icosahedron is the optimum way of forming a closed shell from identical sub-units, the minimum number of identical capsomeres required for each triangular face is 3, which gives 60 for the icosahedron. Many viruses, such as rotavirus, have more than 60 capsomers and appear spherical but they retain this symmetry. To achieve this, the capsomeres at the apices are surrounded by five other capsomeres and are called pentons.
Capsomeres on the triangular faces are surrounded by six others and are called hexons. This structure is composed of a cylinder with a cap at either end.
This membrane is studded with proteins coded for by the viral genome and host genome; the lipid membrane itself and any carbohydrates present originate entirely from the host. The influenza virus and HIV use this strategy. Most enveloped viruses are dependent on the envelope for their infectivity. Some bacteriophages, such as Enterobacteria phage T4 , have a complex structure consisting of an icosahedral head bound to a helical tail, which may have a hexagonal base plate with protruding protein tail fibres, this tail structure acts like a molecular syringe, attaching to the bacterial host and then injecting the viral genome into the cell.
The nucleoid is surrounded by a membrane and two lateral bodies of unknown function, the virus has an outer envelope with a thick layer of protein studded over its surface. The whole virion is slightly pleiomorphic , ranging from ovoid to brick shape.
The capsid appears hexagonal under an electron microscope, therefore the capsid is probably icosahedral;  in , researchers discovered the largest then known virus in samples of water collected from the ocean floor off the coast of Las Cruces, Chile.
Provisionally named Megavirus chilensis, it can be seen with a basic optical microscope. Other archaeal viruses resemble the tailed bacteriophages, and can have multiple tail structures.