@aesthetic_usernamecheck Judging by the scenery, the world building and the roles everyone is playing and the fact this is a historical drama, she's unlikely to have "caught" something from him. Sexual diseases weren't as prevalent back then as they are today. Even when they were prevalent, they were never as bad as what we currently have.
This was because the concept of abstinence and marriage were a bigger thing back then since they were backed by the Church, this kept these kinds of problems in check to a decent degree. It's why she labels the Count a pervert for having several concubines and why it's pointed out that it's weird he even has so many. For the morals of this time, your singular wife should be enough and picking up concubine after concubine reflects on your character as a person.
@Hitspark Yeah they weren't as prevalent as they are now, but the risk is several magnitudes higher. There is no treatment for STDs in the middle ages, while almost every STD can be treated nowadays. So the risk of sleeping with a random man is pretty high.