They must be insured of a way of passing from host to host. They must survive host defense long enough to be guaranteed of making that passage 3. They should maintain the health of the host for as long as possible so they have a greater likelihood of being passed on many times. First point, sex between humans is governed by one of natures basic drives. Second point, most STD have a long residence time in humans before they are eradicated by host defenses.
Gonorrhea is the most frequently reported infectious disease in the United States.
Syphilis has one of the longest residence times of any STD causing disease and increasing disability often for more than 20 years in an individual. Syphilis is the third most common reported infectious disease in the United States. Third point, most STD do not kill their host. Although herpes is a lifelong infection it does not destroy the host. AIDS is the only STD that consistently kills its host and even then requires 10 to 12 years to do so.