If someone were to have an infection or STD and then it got onto the toilet seat then you could potentially get the STD or other infection from sitting on the toilet seat.
But the good news is that it's virtually impossible to get an infection or STD from a toilet seat because the STD or infection would have to leave the toilet seat and get into your body through your vagina or somewhere else if you're a man.
You'd likely have to get the STD or other infection through your mouth if you're a man or possibly through your penis which is unlikely.
As long as you wash your hands with soap and hot water after sitting on the toilet you should be fine.
The STD or other infection will also not live long on the toilet seat either so I would not worry about getting an STD, infection or other disease from the toilet seat as it's nearly impossible to get anything from a toilet seat.