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. These programs periodically check your computer system for the best-known types of viruses.
Some people distinguish between general viruses and worms.
Lion is a dangerous Linux worm that infects computers running Linux. This worm is similar to Linux. Ramen and does not execute on systems running Microsoft Windows. The worm appears to spread using a known TSIG vulnerability discovered early in 2001 to spread. The Evolution of 32-Bit Windows Viruses Chode, Foreskin and BAT911.