Computer viruses hide within other programs or documents and spread as a side-effect of user action (e.g. opening an attachment). They come in many forms, and you don't need to install a program for your computer to be infected. For example, some viruses are spread when you open a word processing document, particularly if you have macros enabled. Once your computer is infected, the virus may attach itself to outgoing emails or may be sent as an email attachment.

A trojan horse is a program that disguises itself as another program. Similar to viruses, these programs are hidden and usually cause an unwanted effect, such as installing a back door in your system that can be used by hackers. They differ from viruses because they typically are not designed to replicate like a virus.

A worm, like a virus, is designed to copy itself from one computer to another, but it does so automatically. First, it takes control of features on the computer that can transport files or information. Once a worm is in your system, it can travel alone. A great danger of worms is their ability to replicate in great volume often by using your email address book.