A virus is a self-replicating piece of software which usually sends itself to other computers via email or the Internet. It does not require human intervention. Its purpose is to either replicate, cause computer damage, or both. It typically comes from infected emails or documents and can either do its damage right away, or be like a ticking time bomb waiting for the special day to activate.
Examples of viruses:
Boot viruses such as Michelangelo and Disk Killer load when the computer reads the disk. This type of virus is extremely difficult to get rid of.
Program viruses attach themselves to the executable programs on the computer and replicate themselves to all executables on the hard drive. Again, these are very difficult to remove. Examples include the Sunday Virus and Cascade Virus.
