“A computer virus is a computer program that can replicate itself and spread from one computer to another. The term "virus" is also commonly but erroneously used to refer to other types of malware, including but not limited to adware and spyware programs that do not have the reproductive ability. A true virus can spread from one computer to another (in some form of executable code) when its host is taken to the target computer; for instance because a user sent it over a network or the Internet, or carried it on a removable medium such as a floppy disk, CD, DVD, or USB drive” http://en.wikipedia.org/wiki/Computer_virus
A virus is a computer program that duplicates itself and can spread from one computer to another. There are 3 different types of viruses, on is the normal virus, the other two are trogen horses and worms.
In schools they could install anti-virus to scan any virus and then delete them.
It is spread via most commonly emails or shared USB drives.