the question said dating, so took the question seriously. I would Not tell anyone immediately because you don't know the outcome of the essential meet and greet. but hopefully you have not taken it to the intimate level if you are just talking/dating, so you can take your time to feel things out before you get too serious and you can know if they are feeling you. so , when you decide to be serious you can let them know and hopefully they will be wise and compassionate enough to let you know the truth about how they feel about you and the situation . but if its like Herpes or genital warts then just tie them down and take it.
Why tell immediately if you don't even know if you will even like that person beyond the first time meeting. You should say something if you are about to have sex tho, so they know what they are getting to so they can protect themselves or opt out of having sex.