The term virus is often used imprecisely to refer to not only viruses, but also Trojan horses (benign-looking programs which infect files on computers in order to do the real damage later) and worms (which propagate and infect other machines). In this document we will refer to all of these as viruses for simplification. The term virus is used to point out the analogy between biological viruses and computer viruses. Like biological viruses, the computer variant can be dangerous, especially if not caught and treated quickly. Some viruses may not do serious damage to your computer. They may display silly messages or change your screen saver. Most viruses, however, were not created with such good intentions, and can do serious damage to your computer. A virus may cause your computer to crash frequently, delete important files, and even infect your computer so that it can no longer boot. Viruses can be passed to other computers whenever files are shared. Worms are highly contagious and will infect other computers without human intervention. (For example, many worms will automatically send themselves as email attachments to everyone in your address book.)