How viruses spread
The most likely virus entry points are email, Internet and network connections, floppy disk drives, and modems or other serial or parallel port connections. In today's increasingly interconnected workplace (Internet, intranet, shared drives, removable drives, and email), virus outbreaks now can spread faster and wider than ever before.
The following are some common ways for a virus to enter your system:
Email attachments
World Wide Web (WWW) sites
FTP traffic from the Internet (file downloads)
Shared network files & network traffic in general
Demonstration software
Pirated software
Shrink-wrapped, production programs (rare)
Computer labs
Electronic bulletin boards (BBS)
Diskette swapping (using other people’s diskettes for carrying data and programs back and forth)