I want to join an online dating service, but I have a health concern...details provided?

Don't mention it unless you are going into a sexual relationship. Just getting to know one another and dating, I would wait until things are further along and get more serious. It's really none of their business unless you are putting them at risk for passing it over to them. You know all the precautions to take. They would need to be informed once things get a lot closer such as sex.