I told my employer about a medical diagnosis I was given and asked him to not tell anybody. It is a very serious condition and I didn't want my family to know. He told several other employees about it and it got back to my family. It's been horrible. My condition does not hinder any part of my job and it was strictly told in confidentiality. What can I do now?

What country?
Generally this doesn't fall under any right to privacy, because you weren't disclosing this as part of something like an insurance claim for medical benefits. If you told your employer, asked him not to tell, and he agreed, then you could possibly sue him for pain and suffering, but as long as you keep your job there, you might have a tough time suing the company. Even if people there treat you differently, an employer can claim that others have a right to know who has a serious condition because of concerns for their health, depending of course on what it is. Even non contagious conditions can pose a risk to others in the workplace, a lawyer could argue. And there's not much you can do about it getting back to family, because you can't prove how it got to them.
You can consult some lawyers but I don't think you will find any takers.
I'm sympathetic - I had a case of plain old Lyme and the doctor told my family when he called the house about something, and I didn't want them to know because I knew they would want me to get rid of my dog! I managed to handle it, but I wasn't happy either.

First learn that honestly esp with family is important. In most illness our family should be aware, esp close family. That is a moral issue , not legal one.

From the legal side, they most likely broke some company policies and you can file a report to the HR about it. Although supervisors you work for, would have had access to know this.