In computer security technology, a virus is a self-replicating program that spreads by inserting copies of itself into other executable code or documents. Thus, a computer virus behaves in a way similar to a biological virus, which spreads by inserting itself into living cells. Extending the analogy, the insertion of the virus into a program is termed infection, and the infected file (or executable code that is not part of a file) is called a host

These viruses can be found as an attachment that will replicate itself by automatically mailing itself to many people in the original victims email address book. Some do not even require opening the attachment, the user only has to view the infected message in the preview pane.

A virus disguised as a computer program. It can not replicate on its own therefore it will prompt user to run the program i.e. a game program.

A virus as a small piece of software that survives by using computer networks and security holes to replicate. The worm virus scans the network for any security holes and then starts replicating from the vulnerable spot.