Why viruses that say they're from you aren't

ZDNet: Why I'm not sending you viruses

Robert Vamosi at ZDNet has written a very accessible article about spoofed return addresses in viruses. Every tech person has had a family member complain to them about an email that says 'You sent me a virus, so you must be infected'. This occurs when viruses use a false return address, an easy programming trick; heck, the command-line mailer in Linux lets you do it, or at least used to.