Most viruses can also replicate themselves.
All computer viruses are man-made. A simple virus that can make a copy of itself over and over is relatively easy to produce. Even such a simple virus is dangerous because it will quickly use all available memory and bring the system to a halt. An even more dangerous type of virus is one capable of transmitting itself across networks and bypassing security systems. A worm is a special type of virus that can replicate itself and use memory, but cannot attach itself to other programs.