If someone repeates a rumor they heard about someone's medical condition, is this a hipaa violation? And can the person repeating that rumor be sued for slander?

No. A violation occurs when someone working in a health care facility providing care for an individual reveals information about the treatment. The person who was the source of the rumor might be guilty of a violation, but not someone spreading the rumor.

Not unless they got that information from your private medical documents.

No that is called being a gosip and against the Ten Commandments but not against mans law.