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An adenovirus An adenovirus The smallest living things are viruses. They are “alive” only because they can produce more of their kind if they invade another living thing. Viruses cannot reproduce on their own. They get into another living cell, the host cell, and take over its life processes to make more copies of themselves. In the process they destroy the host cell. A typical virus has an outer shell or coat made of proteins. Inside is a length of genetic material, either DNA or RNA, a single-strand version of DNA. Viruses are incredibly tiny: the smallest ones studied are 20 billionths of a metre in diameter. Most viruses are too small even to be observed through an optical microscope, and can only be viewed through electron microscopes. The study of viruses is called virology.

Bacteriophages, a large virus that attacks bacteriaBacteriophages, a large virus that attacks bacteria

Kinds of virus

Different viruses are shaped like bricks, rods, golf balls and even space rockets. Many can exist in their non-living form for years and be frozen solid, boiled or made into crystals—yet still come alive when host cells are available. 

Viruses cause diseases in plants, animals and people. These include colds, flu, measles and AIDS (caused by the Human Immunodeficiency Virus, HIV).

Bacteriophages attacking a bacteriaBacteriophages attacking a bacteria


Viruses were first discovered in the 1890s.

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