Viruses are tiny organisms which nearly always produce diseases in animals and plants. A viruses are parasites which can live only in other life forms. Technically, they are not living creatures at all, because they can reproduce and carry out the normal processes of life only when they are inside a cell, and forming a part of the cell's structure. Viruses invade cells and take over the genetic material (DNA and RNA), changing its function so that the whole cell becomes a factory' producing viruses. Eventually the cell bursts and dies, releasing the new viruses to spread Because the function of the cell is affected, infection with viruses nearly always causes disease. The body's usual defences cannot easily fight the virus, because once the virus is inside the cell, the body's immune system is unable to recognize the invader. Antibodies can attack the virus only when it bursts out of the cell ready to infect other cells. Drugs such as antibiotics do not work against viruses, and the immune system must be relied upon to fight the infection. The HIV virus, which can cause AIDS, is particularly dangerous because it infects and kills the cells of the immune system which normally fight diseases. Virus particles, or virions, are very tiny. Viral diseases are usually spread by viruses carried in water droplets in the air, which are inhaled into the lungs. Colds and influenza are common infections caused by viruses