Without knowing they had it...or without telling they had it?
How irresponsible is it to engage in high-risk behavior with a stranger not knowing if you are infected with a disease that could be transmitted? These deviants get tested for STD's once every three months at most, and in between checkups they probably have several dozen hookups. That makes the checkups meaningless regarding "protecting others," and is mostly to catch a disease and treat it for self-preservation.
It is the ultimate selfish lifestyle.