What are Viruses ???

Viruses term is a latin word which means venom or poison.
Viruses are minute microorganisms much smaller than bacteria
(size ranges from 0.015 – 0.2 micron) and can be seen only with
the help of an electron microscope.
M.W. Beijerinck (1898) demonstrated that the extract of infected
plants of tobacco causes infection in healthy plants and called
the fluid as contagium vivum fluidum.
W.M. Stanley (1935) showed that viruses could be crystallised and crystals consist of proteins.

Characteristics of Viruses

• They are not free living.
• They grow and multiply only inside other living cells.



• Outside a living system, a virus is like a non-living substance. It neither respires nor reproduces outside the host.
• They are a link between living and non-living things.
• A virus is a nucleoprotein and the genetic material is infectious.
• The protein coat is called capsid that protects the DNA/RNA.
• Antibiotics have no effect on viruses as they do not have a metabolism of their own.
• Viruses found in plants are known as plant viruses. Similarly, they are categorised as animal viruses or bacterial viruses or bacteriophages.

Disease Causing Viruses (Harmful Viruses)

Many human diseases like influenza, common cold, measles,
mumps, chicken pox, rabies, etc., are caused by viruses. One of the most fatal disease caused by a HIV (Human Immunodeficiency Virus) virus is AIDS (Acquired Immuno Deficiency Syndrome). 
It spreads through sexual contact with an infected person.
In 1971, T.O. Diener discovered very simple smallest infections
agents called viroids. They contain only RNA, capsid is absent.