Virus (Latin, “poison”) is any of a number of organic entities consisting simply of genetic material surrounded by a protective coat. The term “virus” was first used in the 1890s to describe agents that caused diseases but were smaller than bacteria. By itself a virus is a lifeless form, but within living cells it can replicate many times and harm its host in the process. Viruses infect all cellular life. The first known virus, tobacco mosaic virus, was discovered by Martinus Beijerinck in 1899, and now more than 5,000 types of virus have been described. The study of viruses is known as virology, and is a branch of microbiology.

There are three types of classification for virus:
· Lwoff’s Scheme for Classification
· Baltimore’s Scheme for Classification
· ICTV classification.

Lwoff’s Classification is based on
- shared properties
- nucleic acid of the virus
- Symmetry of capsid
- Presence / absence of envelope
- Dimensions of virion and capsid

Baltimore classification is classification of virus based on viral genome and its relationship to mRNA.
There are seven groups of viruses and each will be classified differently:
I: dsDNA virusesII: ssDNA virusesIII: dsRNA virusesIV: (+)ssRNA virusesV: (−)ssRNA virusesVI: ssRNA-RT virusesVII: dsDNA-RT viruses

ICTV classification
- Nature of genome & sequence relatedness
- Virus structure (symmetry, shape, etc.)
- Natural host range
- Cell and tissue tropism
- Pathogenicity and cytopathology
- Mode of transmission
- Physicochemical properties of virions
- Antigenic properties of viral proteins

Viruses spread in many ways; different species of virus use different methods. For example, plant viruses are often transmitted from plant to plant by insects that feed on sap, such as aphids, while animal viruses can be carried by blood-sucking insects. These disease-bearing organisms are known as vectors. Influenza viruses are spread by coughing and sneezing, and others such as norovirus, are transmitted by the faecal-oral route, when they contaminate hands, food or water. Rotavirus is often spread by direct contact with infected children. HIV is one of several viruses that are transmitted through sex.
(http://uk.encarta.msn.com/encyclopedia_761575740/Virus_(biology).html)
(http://en.wikipedia.org/wiki/Virus)

Virus reproduces itself by entering the cell and replicate with the six phases:
1. Attachment
2. Penetration
3. Uncoating
4. Replication & Expression
5. Assembly
6. Release

In this blog, we will concentrate on the six viruses Herpesviridae, Poxviridae, Hepdnaviridae, Flavivirdae, Picornaviridae and Orthomyxoviridae.