Such viruses are called retroviruses. The coat of a virus consists of individual proteins that give the virus its shape. This coat protects the nucleic acid and helps the DNA or RNA get inside a cell. Some viruses have an additional outer membrane that provides further protection. How a virus infects an organism.
Most viruses reproduce in specific cells of certain organisms.
For example, viruses that cause colds reproduce in cells of the human respiratory tract. Viruses cannot live outside their particular cells. They must be carried into the organism by air currents or some other means, and then transported by body fluids to the cells. When a virus comes into contact with a cell that it can enter, it attaches itself to the cell at areas called receptors. Chemicals in the receptors bind the virus to the cell and help bring it or its nucleic acid inside.