A typical virus has two basic parts, a core of a nucleic acid and an outer coat of protein. Some RNA viruses contain an enzyme called reverse transcriptase, which converts virus RNA to a DNA copy inside cells. 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.