I slept with someone last night, kinda a one nighter, i want to go for testing tomorrow. Is it useless to go so soon?

For an entire night, in the best hotel in the world. The std is…one that is incurable and makes your crotch itch like crazy.

I’m somewhat confused about the spread and creation of stds? If it’s impossible for me and the first person who say both have never had partners. Why would the second person get an std? or the third?
Where does it start from? assuming everyone was born without the std and not from their parent?

If you sleep with someone with a std. numerous times unprotected and you find out they have an std, is it possible you won’t contract it ?