A virus is a small infectious micro organism; it is much smaller than a fungus or bacterium. It needs a cell to replicate. The virus attaches to a cell, make entrance in it and releases inside the cell its DNA or RNA, takes control of the cell and forces it to replicate the virus. The infected cell dies and releases new viruses which go on to infect other cells.

Some viruses do not kill the host cells, but instead alter the functions and sometimes the infected cell loses control over normal cell division and becomes cancerous. Some viruses have their genetic material in the host cell, where it remains idle for an extended time, the so called latent infection. When the cell is disturbed, the virus may begin replicating again and causes disease.

Viruses infect usually one particular type of cell; i. e. cold viruses infect only cells of the upper respiratory tract. Some infect plants or animals, some human beings, but they commonly infect infants and children.

Viruses are spread or transmitted in several ways, some are inhaled, some swallowed and others are spread by the bites of insects and other parasites, for example, mosquitoes and ticks. The very dangerous are the infections which are spread sexually, e.g. HIV infection. Safer sex is the only alternative.