A virus is a unit of code whose purpose is to reproduce additional, similar code units. To work, a virus needs a host, such as a file, e-mail, or program. Like a virus, a worm is a unit of code that is coded to reproduce additional, similar code units, but that unlike a virus does not need a host. Viruses and worms primarily show up during file transfers between clients or when URLs are sent from other users. If a virus is on your computer, it can, for example, use your identity and send instant messages on your behalf.