Which sexually transmitted disease is so common that 50 percent of sexually active adults contract it at some point in their lifetime?

I think the answer is Herpes Simplex.
Herpes simplex is a viral disease caused by the herpes simplex virus. Infections are categorized based on the body part infected.  There are two types: One infects the area around the mouth known as the herpes simplex virus 1, while the other infects genital areas called the herpes simplex 2

Land marks is ruler of the world i just wanna let you kno that you are really smart