Server Security: Keeping Your Data Safe Page 6
By Karl Magsig
Computer viruses are spread two basic ways. One is via e-mail, sending and receiving e-mail with infected attachments, and the other is by transmitting infected files, either by downloading infected files from the Internet or by copying files from an infected computer onto a clean one. (This can happen in any file-copy scenario, e.g., from floppy disks to shared network resources.)
When a virus is transmitted via e-mail, it comes in the form of an attachment to an e-mail message, normally from a known colleague, someone who would have you listed in her address book. These e-mail viruses will generally send themselves to everyone listed in the infected computer's address book (the address book of the default e-mail client program). The e-mail client does need to be running for this to happen, but in most all cases it is, since this is how the virus was received in the first place. Some of them will automatically run when they are received in specific e-mail clients. These attachments usually come in the form of a VisualBasic Script, which most Microsoft e-mail clients are configured to run automatically when the message is opened. Thus, you don't have to double-click the e-mail attachment to infect your computer with the virus.
What can be done about this, short of deleting all e-mail attachments? You can run an e-mail virus scanner.