If I were creating viruses, for a living or whatever, here's the virus I would create. It would be more apt to thrive in a business environment than a home computer, so it might have a short life since a lot of companies have good virus programs. But, here's how it would work.
First, it would look through your inbox and figure out who you correspond with most often. Then, it would scan subjects. Thirdly, it would look to see what attachments were most commonly used. Then, it would respond to the e-mail in your inbox with a short message like "I'm not sure I like where this is heading. I've attached some of my own figures." or "I'd go a different route. Take a look at this idea." or "Oh... before I forget, check out this out. My friend sent it to me. It's pretty funny."... something like that. And sign their name at the bottom.
Maybe even a variant that identifies e-mails that were forwarded multiple times and then pass it on to a bunch of people in the address book. Then it would just look like they were forwarding an annoying joke or something.
It would include an attachment that when opened, appeared to be blank. You would be infected at that point, but you would just think your colleague or friend sent you a blank e-mail and you'd write back going "Uh, what did you send me?"
I think it would spread like wildfire.