(UPDATE) I was pointed to this website and article. Apparently, there is really a worm going around using the instant messaging applications like Yahoo! Messenger. The virus is called Sohanad and it has spawned variants.
It spreads via the instant messaging applications Yahoo! Messenger, AOL Instant Messenger (AIM), and Windows Live Messenger. It sends an instant message to all contacts of an affected user. The said message contains a link that when accessed, downloads and executes a copy of itself.
Yugatech recently wrote that a supposed Yahoo! Messenger virus is on the loose, perhaps infecting some innocent users. But I’ve done some online research and found some anecdotal evidence. I’ve also looked at some anti-virus websites, and found this on Trend Micro:
Yahoo Instant Messenger Hoax
This hoax spreads via both email and Yahoo! Messenger (YM), the instant messaging service from Yahoo!.
It advises users to be wary of IM messages from specific YM accounts. It claims that files from these IM messages are viruses.
This claim has no factual basis. Users are encouraged to ignore these warnings.
If somebody by name email@example.com adds you. dont accept it. Its a virus. Tell everyone on ur bulletin because if somebody on ur list adds them, u get the virus too. Tell everyone on your list not to open anything from angell11, tewwtuler, and sassybitch. It is a hard drive killer and a very horrible virus. pass this letter to everyone on your buddy list. We need to find out who is really using these accounts. Sorry for the inconvienience. Sincerely, Director of Yahoo Services, tanwir2001.
Right click on the group name of your buddy list and click Send Message to All