Viruses are the smallest of all infectious agents.
Because they themselves lack the ability to make copies of DNA and RNA,they can only reproduce when they are inside a living cell. Viruses work by injecting their genetic material into a cell, called the host cell. The genetic material of the virus then directs the cell to reproduce more of that same virus, that is to manufacture the same combination of protein and genetic material. As the viruses grow, they rob the cell of its nutrients and move on to infect other cells. In some cases, they actually kill the cell by bursting it.