George and Izzie find out about Mer and McDreamy while Cristina faces a test she's not quite prepared for.

An outbreak of a sexually transmitted disease affects several members of the hospital staff, prompting Chief Webber to call a meeting. Meanwhile, the chief reluctantly confides to Dr. Shepherd about his own medical concerns; Izzie and Cristina are worried by a patient's wife and daughter who are at odds over his care; Burke treats Bill, a college buddy whose medical diagnosis raises some questions regarding his wife's actions; and, just as Meredith and Derek grow more intimate, a bombshell is dropped, on the season finale of Grey's Anatomy.