The name virus has it origin from the Latin “virus” meaning poison or slime. Viruses are so extremely small; they can only be seen under an electron microscope. They inspect living cells and multiply inside them and then kill the cell. When a virus is outside a living cell, it is called a virion.

A virion is inactive or inert. That is, it cannot reproduce, cannot respire, etc. Viruses do not have a nucleus, cytoplasm or cell membrane. Viruses can be crystallized when not found in a living cell. Viruses have not yet been classified into any of the five kingdoms.

GENERAL CHARACTERISTICS OF VIRUS

• Viruses are extremely small.
• Viruses can only be seen with the help of an electron microscope
• Viruses are inactive or inert and become active only when they are in a living cell.
• They have either RNA (Ribo Nucleic Acid) or DNA (Deoxy-ribo Nucleic Acid) as their genetic material but never both.
• A complete viral particle or virion consist of a capside or protein coat which protect the nucleic aside molecule (RNA or DNA) which is contained inside the virus.
• They are highly specific in their choice of host.
• Viruses do not have organelles like plant and animal cells.
• They have no nucleus, no cytoplasm and no membrane.
• All viruses are obligate intracellular parasites. That is, they are infectious particles that can function only inside a cell.

STRUCTURE OF A VIRUS (BACTERIOPHAGE)

A virus has a relatively simple structure. It has nucleic acid surrounded by a protein coat. This protein coat has two functions

• To protect the delicate structure of nucleic acid in the centre of the virus.
• It is a means of entry of nucleic acid into the host cell.

Viruses are spherical in shape and very small in size. The small pox virus is one of the largest and that of poliomyelitis is one of the smallest. The T₂  bacteria cells invade bacteria cell.