Argh. Stupid email and antivirus programs.
So. Apparently there's a race condition wherein my antivirus program will detect viruses in the Inbox of my email program and swipe it out from under the email program before the email program has a chance to move the attachment out into a separate file. The proximate result is that the email program then tries to do something to the Inbox that causes the antivirus program to be unable to restore it back, and the ultimate result is that my Inbox goes "poof".
So, I've just spent the last 45 minutes restoring it from backup (a month old), and going through my secondary email account and finding all the lost emails since then and bouncing them back to my primary email account. Twice, because the first time I did that, I bounced them to my main address (which goes to both accounts), and the mailer decided that was a mail loop and rejected them all. Unfortunately, my secondary email account is packed full of spam because I don't have filters set up on it, so this took a while.
I am now making a more recent backup of my email, and also told my antivirus program to ignore my Inbox.