Unfortunately, this sort of harassment  is not  illegal. That is, there is nothing illegal about telling  someone that another person poses a risk of disease, assuming that the person does in fact have that disease. There are no grounds to sue her; the law does not regulate what people say, so long as they do not lie about facts specific to a person (that is, if you did not have the disease and she was lying and saying you did, that could be defamation; but the fact that she is incorrect in the risks of hep C is not defamation and is not actionable).