A virus is a small infectious agent that replicates only
inside the living cells of other organisms. Viruses can infect
all types of life forms, from animals and plants to microorganisms,
including bacteria and archaea.
Since Dmitri
Ivanovsky's 1892 article describing a non-bacterial pathogen infecting
tobacco plants, and the discovery of the tobacco mosaic virus by Martinus
Beijerinck in 1898. about 5,000 virus species have been
described in detail, although there are millions of different
types. Viruses are found in almost every ecosystem on Earth and
are the most abundant type of biological entity. The study of viruses is
known as virology, a sub-speciality of microbiology.While not inside an infected cell or in the process of infecting a cell, viruses exist in the form of independent particles. These viral particles, also known as virions, consist of two or three parts: (i) the genetic material made from either DNA or RNA, long molecules that carry genetic information; (ii) a protein coat that surrounds and protects the genetic material; and in some cases (iii) an envelope of lipidsthat surrounds the protein coat when they are outside a cell. The shapes of these virus particles range from simple helical and icosahedralforms for some virus species to more complex structures for others. Most virus species have virions that are too small to be seen with anoptical microscope. The average virion is about one one-hundredth the size of the average bacterium.
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