HOW INFECTIONS OCCUR
A virus can infect a computer in a number of ways. It can arrive on a floppy disk or inside an e-mail message. It can piggyback on files downloaded from the World Wide Web or from an Internet service used to share music and movies. Or it can exploit flaws in the way computers exchange data over a network. So-called blended-threat viruses spread via multiple methods at the same time. Some blended-threat viruses, for instance, spread via email but also propagate by exploiting flaws in an operat- ing system.
Traditionally, even if a virus found its way onto a comput- er, it could not actually infect the machine—or propagate to other machines—unless the user was somehow fooled into executing the virus by opening it and running it just as one would run a legitimate program. But a new breed of computer virus can infect machines and spread to others entirely on its own. Simply by connecting a computer to a network, the computer owner runs the risk of infection. Because the Internet connects computers around the world, viruses can spread from one end of the globe to the other in a matter of minutes.