I think that that's one of the most dangerous pieces of advice you see
around, because it implies that it's safe to do the opposite--open
attachments from friends and relatives. But many viruses spread by sending
themselves to everyone in the infected party's address book, so attachments
received from friends are perhaps the *most* risky to open.

Even if the attachment legitimately comes from a friend, it can contain a
virus. I'm not suggesting that a friend is likely to send you a virus on
purpose, but if the friend is infected without realizing it, any attachment
he sends you is likely to also be infected.

I never open executable attachments at all, except from a *very* few trusted
sources, and then only when I'm expecting them.