This is one that came up at work that we wrestled with for awhile before a very odd solution arose and saved us.

You work in a hospital.

One day you go to a run down little greasy spoon diner downtown for a burger. The place looks rough but usually has good food. While there you spot a patient from your hospital working there. You know for a fact, beyond any shadow of any doubt, that he has an active tuberculosis infection and that he is noncompliant with his course of medicine.
Now, this patient's medical privacy is protected by the law. It is illegal for you to tell the owner of the restaurant or the county health department (who inspects restaurants to make sure that they are run in healthy ways) about his medical condition.
On the other hand, a concern for public health and simple decency makes you want to call the county.
What is the right thing to do in this situation?