The good news is that you´re unlikely to contract a disease merely by sitting on a pathogen-covered toilet. Neither viruses like influenza nor the bacteria responsible for illnesses such as strep throat are dangerous unless they come in contact with the mucus membranes
-something easily prevented by washing dirty hands with soap before touching your mouth or eyes (which you do already, right?). Most sexually transmitted diseases cannot survive once exposed to air (exceptions are the herpes virus, which can live for a few hours, and hepatitis B, which can linger for seven days). To catch a disease, the seated party would have to have some sort of break in the skin to allow the virus to enter. So if your bum is flawless and you don´t mind the yuck factor, go ahead and take a seat.