Most viruses can also replicate themselves.
All computer viruses are manmade. A simple virus that can make a copy of itself over and over again 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. In most cases those virus warnings are hoaxes and forewarding that email is just playing into the hands of those crafty pranksters.