That means millions of AOL users can't chat online with the folks on Yahoo Instant Messenger or MSN Messenger, and vice versa.

FORBES: Instant Headache

If AOL's services work together more fully, "it opens the door for us to argue for interoperability, " Brian Park, a senior instant messenger producer at Yahoo, said in Tuesday's edition of the Wall Street Journal.

CNN: AOL tests linking of instant messenger services

Variations on the Yahos malware have been infecting users for years, spreading initially via instant messenger platforms like AIM and Yahoo!

FORBES: Facebook Helps FBI Smash 11-Million-Machine 'Butterfly' Botnet