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These are so minute to be seen with light microscope. They
can be studied only with the help of electronic microscope.
Many human diseases e/g/ small fox, poliomyclitis, mumips,
yellow fever influenza, measles are all due to virus
infection. Certain diseases of plants characterised by
mottling, rolling, and wrinkling of leaves. These are also
caused by viruses.
In 1955 Santley succeeded in obtaining the tobacco mosaic
virus in the form of definite crystals. The particle of
virus show a good range in shape. They may be hexagonal,
spherical or rod like. These is just a core of nucleic acid
surrounded by a sheath of protein. Nucleus, cell membrane
and cytoplasm are absent.. These viruses are not free
living. These viruses grow in cells of living being. Viruses
possess some chief characteristics which place them in
loving being such as property of Reproduction viruses can
mutate also. Outside the body of loving begin it is dead
live a stone. It neither respires or reproduces. It
reproduces in large number inside the cells living beings.
Crystals of viruses can be stored in bottles just like salt
or sugar, So we can say that these viruses form a link
between the loving and non-living
Viruses mostly spread by contact. Insects, betles, leaf
hoppers transfer them from diseased to healthy plant. Virus
diseases of plants can be controlled by
a) Reducing the insect vectors.
b) By sowing diseases free seeds.
c) By using varieties of plants resistant to a particular virus.
The virus caused influenza and cold. This is also spread by
contact, coughing or spitting. Polio virus is transmitted by
way of infected faeces files also acts as a carrier of these
diseases. Some of human diseases which are caused by viruses
can be cured by vaccination. Vaccination is injecting of a
weaknened virus in the body which causes the formation of
antibodies that counteract the virus make the individual immune.
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