You open the attachment, and ... BOOM! You just got hit with a worm or Trojan Horse. Why would someone you know do this to you? Because some things are not as they appear. You've read about this: the worm that gets onto someone's computer, and then emails itself to everyone in that person's address book. Each of those people gets an email from someone he knows and trusts, and each of those trusted emails contains the worm, which propagates itself like the ripples from a stone thrown into a still pond.