Yahoo! Specific Virus?

I have recently been having trouble with my Yahoo account. Stored emails have disappeared (sent and received). An associate of mine (also on Yahoo!) had her entire account vanish — no access or anything. I am not experiencing this difficulty with any other email accounts (Hotmail, Optonline) and was wondering if anyone might know why this is happening. Have run virus software, but nothing appeared on my computer. Is there a virus/tojan/worm that affects Yahoo servers specifically, and how can I stop this from happening anymore? Thanks.