How do viruses and worms spread?
The earliest viruses spread by floppy disk. Someone would place their disk in an infected machine and the virus would append itself to any programs, or the disk’s boot sector. Then when that disk was used on another machine, the virus spread to it. Many worms, like Code Red, or MS Blast, were able to exploit security holes or software bugs to insert themselves onto other computers. Early versions of e-mail worms relied on a flaw in Microsoft Outlook and Outlook Express that cause the worm to activate as soon as the message was viewed. Many e-mail worms still try to exploit older, known security flaws, but most simply try to trick you by appearing to be from a friend, your ISP, or from some other official seeming source.
