Numbers
Viruses
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Viruses are infectious agents of small size that can replicate and multiply only in the living cells of animals, plants, or bacteria. The name comes from a Latin word meaning ‘poison’ or ‘slimy liquid’. Note that viruses are not alive: they lack any form of energy or metabolism and cannot replicate or evolve. Viruses are, in short, parasites: they depend on the host cell for almost all of their life-sustaining functions. The study of viruses is known as virology, a sub-speciality of microbiology.
1892, 1898: The year the first virus (but not yet identified as such) was discovered. Dmitri Ivanovsky, a Russian botanist, wrote an article describing a ‘non-bacterial pathogen’ infecting tobacco plants. 1898: The year the tobacco mosaic virus was discovered, by Martinus Willem Beijerinck, a Dutch microbiologist and botanist. 1901: The year the first human virus (yellow fever virus) was discovered. Viruses are found in almost every ecosystem on Earth, are the most numerous type of biological entity, and can infect all types of life forms: animals, plants, and other microorganisms such as bacteria and archaea. 1933: The year the first human influenza virus was isolated. 2: The number of classes of viruses. Viruses can contain either DNA or RNA as the basis for their genetic material; in turn, these can be single-stranded or double-stranded.
Millions: The estimated number of types of viruses on the planet. 5,000: The number of virus species that have been described in detail. 10-100: The number of genes viruses have, depending on the virus. Compare this to the figure 30,000, the number of genes in the human body. 20-400: The range of diameters in nanometres (nm or 10-9 m) of most viruses. The smallest known viruses are circoviruses (20 nm in diameter), while the largest are mimiviruses (400 nm in diameter). Coronaviruses are approximately 120 nm wide. 10: The number of years the Human Immunodeficiency Virus (HIV, the virus that causes AIDS) can remain dormant, ie without causing any apparent changes, inside the host’s body.
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