Legal Question in Civil Litigation in California

You can if the co-employee only learned about it because of some information they were to keep confidential, but if they learned about your medical condition in some other fashion, they would not be breaching any duty they owe you. Your employer might be negligent in allowing the information to become public; in general, employers are liable for the acts of employees committed in the course and scope of their normal employment duties. The employer might try to argue that it is an industrial injury [Workers' Compensation] but since it is basically a mental/emotional injury it should fall outside the WC framework, but the case law used to be not that clear.

To summarize Mr. Shers' long-winded answer: If the co-worker got the information from confidential company records, you can sue them and the company. If the co-worker got the information from you, you should have kept your mouth shut; you have no recourse.