There is a virus going around that spreads itself by going into your mail and replying to already read mail. There is nothing in the subject line that would give you a clue that it may be a virus. I recieved 2 infected e-mails, In both of them the subject was just a reply to something I had written. Both e-mails were from members of this BB who were infected.
If you receive an email that is a reply and has an attachment, delete it.