Viruses are small infectious agents that can only replicate inside host cells. They contain either DNA or RNA and use the host cell's machinery to produce new viral components and assemble new virus particles. Viruses cause a variety of infectious diseases in plants, animals, and humans. Some early evidence of viral diseases include depictions of the Pharaoh Siptah who showed signs of polio and the mummy of Ramses V which had pockmarks indicating smallpox. The field of virology began in the late 19th century with the discoveries that certain plant and animal diseases could be transmitted through filters, indicating an unknown infectious agent smaller than bacteria. This led to the identification and characterization of many important human and animal

1. 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 aheiroglyph 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.

2. 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

3. 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.