Worms can do all the nasty things that viruses can do, but they don’t rely on a host file or application to be able to run. They are, in effect, completely contained programs that can wreak havoc and copy themselves without a user having to open an infected file or application. Worms can often spread over networks by exploiting security vulnerabilities in individual operating systems and computer configurations. In addition to exploiting security holes on a computer, worms can spread by email (and, less often, instant messaging programs) as a file. In these cases, the file itself generally doesn’t pose much danger unless it is opened, but once opened it infects the computer.