They must be ensured of a way of passing from host to host. They must survive host defenses 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. Sex between humans is governed by one of nature's basic drives. Most STDs 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. Most STDs do not kill their host. Although herpes is a lifelong infection, it does not destroy the host. Syphilis may ultimately destroy its host, but only more than 20 years after infection. AIDS is the only STD that consistently kills its host and even then requires 10 to 12 years to do so.