A program or piece of code that is loaded onto your computer without your knowledge and runs against your wishes.
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. These programs periodically check your computer system for the best-known types of viruses.