Virus are submicroscopic particles those cannot be seen even by microscope. Virus are infectious particles. Virus do not have their own protein mechanieries. They fully depend on host for protein synthesis.

1. What is a Virus?

2. Introduction
● Virus are submicroscopic particles those cannot be seen even by microscope.
● Virus are infectious particles.
● Virus do not have their own protein mechanieries. They fully depend on host
for protein synthesis.

3. History
● It was first identified by Russian biologist Dmitri Ivanovsky.
● The first virus identified was tobacco mosaic virus.
● Tobacco mosaic virus is a plant virus and causes diseases in plants only.
● In the early 20th century English bacteriologist Frederick Twort discovered a
group of virus those infect bacteria, now called as bacteriophages.
● First image of virus was obtained in 1931 on the invention of electronic
microscope.

4. Classification
● In 1966, the international committee on Taxonomy of virus was formed.

5. ICTV Classification
The ICTV developed the current classification system and wrote guidelines that
put a greater weight on certain virus properties to maintain family uniformity. A
unified taxonomy (a universal system for classifying viruses) has been
established. Only a small part of the total diversity of viruses has been
studied.[148] As of 2020, 6 realms, 10 kingdoms, 17 phyla, 2 subphyla, 39
classes, 59 orders, 8 suborders, 189 families, 136 subfamilies, 2,224 genera, 70
subgenera, and 9,110 species of viruses have been defined by the ICTV.

6. Baltimore Classification
The Nobel Prize-winning biologist David Baltimore devised the Baltimore
classification system.[43][149] The ICTV classification system is used in
conjunction with the Baltimore classification system in modern virus classification.[

7. The Baltimore classification of viruses is based on the mechanism of mRNA
production. Viruses must generate mRNAs from their genomes to produce
proteins and replicate themselves, but different mechanisms are used to achieve
this in each virus family. Viral genomes may be single-stranded (ss) or double-
stranded (ds), RNA or DNA, and may or may not use reverse transcriptase (RT).
In addition, ssRNA viruses may be either sense (+) or antisense (−). This
classification places viruses into seven groups:

8. I: dsDNA viruses (e.g. Adenoviruses, Herpesviruses, Poxviruses)
II: ssDNA viruses (+ strand or "sense") DNA (e.g. Parvoviruses)
III: dsRNA viruses (e.g. Reoviruses)
IV: (+)ssRNA viruses (+ strand or sense) RNA (e.g. Coronaviruses, Picornaviruses,
Togaviruses)
V: (−)ssRNA viruses (− strand or antisense) RNA (e.g. Orthomyxoviruses,
Rhabdoviruses)
VI: ssRNA-RT viruses (+ strand or sense) RNA with DNA intermediate in life-cycle
(e.g. Retroviruses)
VII: dsDNA-RT viruses DNA with RNA intermediate in life-cycle (e.g.
Hepadnaviruses)
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