In the case of boot sector viruses, it is the boot sector of the floppy disk or hard drive that is infected. While the primary purpose may be to replicate, some viruses also incorporate a payload - often malicious. Today, over 54,000 viruses are known to exist. Where once viruses reigned supreme, today combination threats are increasingly prevalent. These threats combine Internet worms, viruses, and Trojans in an infectious mass-emailer.
Worms make copies of themselves over and over again.
Today's worms are referred to as Internet worms or email worms, as they send copies of themselves to recipients in infected users' address books. If you follow this usage policy, you have nothing to fear from email-transmitted viruses. If you get mail with attachments from someone you do know, don't open the attachment if you aren't expecting an attachment from them. Don't open it if it has and .exe extension. Don't open it if it has a double extension.