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People - A virus is a small infectious agent that replicates only inside the living cells of other organisms. Viruses can infect all types of life forms, from animals and plants to bacteria and archaea. http://en.wikipedia.org/wiki/Virus 

Computer - A computer virus is a program or piece of code that is loaded onto your computer without your knowledge and runs against your wishes. Viruses can also replicate themselves. All computerviruses are man-made. 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 availablememory and bring the system to a halt. An even more dangerous type of virus is one capable of transmitting itself across networksand bypassing security systems. http://www.webopedia.com/TERM/V/virus.html