Definition of Viruses

Viruses are very small, infectious, obligate intracellular molecular parasites, which do not respire, move or grow. The virus genome is composed either of DNA or RNA and directs the viral replication by the synthesis of virion components within an appropriate host cell. Progeny viruses are formed by de novo assembly from newly synthesized components within the host cell.

Similar Particles

Viroids are small (200-400nt), circular RNA molecules with a rod-like secondary structure which possess no capsid or envelope which are associated with certain plant diseases. Their replication strategy like that of viruses - they are obligate intracellular parasites.
Virusoids are satellite, viroid-like molecules, somewhat larger than viroids (e.g. approximately 1000nt) which are dependent on the presence of virus replication for multiplication (hence 'satellite'), they are packaged into virus capsids as passengers.
Prions are rather ill-defined infectious agents believed to consist of a single type of protein molecule with no nucleic acid component. Confusion arises from the fact that the prion protein & the gene which encodes it are also found in normal 'uninfected' cells. These agents are associated with diseases such as Creutzfeldt-Jakob disease in humans, scrapie in sheep & bovine spongiform encephalopathy (BSE) in cattle.

Origines of Viruses

Viral infections in the old Egypt

One of the first written record of a virus infection consists of a heiroglyph from Memphis, the capital of ancient Egypt, drawn in approximately 1400 B.C, which depicts Siptah. After a short rule of three years of his father Amenmesse, who seized the throne of Egypt from the previous Pharaoh Seti II, Siptah became the seventh egyptian king of the 19th dynasty, ruling for about six and a half years (from 1199 to 1192 B.C). Judging from his mummy, he died at about 20 years of age. The body’s deformed left leg suggests that Siptah suffered from aneuromuscular disease(poliomyelitis). At some time during the 21st dynasty, Siptah’s sarcophagus and mummy were placed in Amenophis II’s tomb. The funeral temple itself, situated on the edges of the area’s fertile land, was never completed.

A photo of the unwrapped mummy of Ramses V, egyptian pharao from 1147 to1143 BC, shows pockmarks from the smallpox virus that attacked and probably killed Egypt's ruler, who died around 1151 B.C.

The Beginning of Virology

The generally recognised beginning of Virology is a paper presented to the St. Petersburg Academy of Science on the 12th February 1892 by Dmitri Iwanowsk (1864-1920), a Russian botanist. He showed that extracts from diseased tobacco plants could transmit disease to other plants after passage through ceramic filters fine enough to retain the smallest known bacteria.

Six years later in Holland, Martinus Beijernick (1851-1931) confirmed Iwanowski'sresults on tobacco mosaic virus. He developed with the terme "contagium vivum fluidum" ('soluble living germ') as first the idea of the virus. Agents that pass through filters that retain bacteria came to be called ultrafilterable viruses, apprprating the term virus from the LAtin for "poison".

The same year (1898), the German scientists Friedrich Loeffler (1852-1915) and Paul Frosch, both former students and assistants of Robert Koch (1843-1910), observed that a similar agent was responsible for foot-and-mouth disease. In spite of these findings, there was resistance to the idea that these mysterious agents might have anything to do with human diseases.

These pioneering work on tabacco mosaic- and foot-and-mouth disease virus was followed by the identification of viruses associated with specific diseases in many other organisms.

Poliovirus was discovered by Karl Landsteiner in 1909. Before the discovery of the infectious nature of the disease, the paralytic aspect was considered to be its characteristic feature, as documented by the denomination "infantile paralysis." Although its infectious nature was long hypothesized, Ivar Wickman was the first to clearly show the infectious nature of polio after an epidemic in Sweden in 1905

In 1911, Peyton Rous (1879-1970) was able to prove that some «spontaneous» chicken tumours, to all appearances classical neoplasms, are actually started off and driven by viruses (Rous sarcoma virus) which determine their forms as well. These findings led him to spend several years trying to get similar agents from mouse cancers; but, failing in this, he left off working with tumours in 1915, turning instead to the study of other problems in physiological pathology.

Bacterial viruses were first described by Frederick Twort (in 1915; left) and Felix d'Hérelle (in 1917; right). D'Hérelle named them bacteriophages because of their ability to lyse bacteria on the surface of agar plates.Rapidely many scientists utilized these viruses as model systems to investigate many aspects of virology, including virus structure, genetics, and replication.