Sexually transmitted pathogens take on secretive and even sinister strategies. The underlying reason is that people have sex with new partners less frequently than they come within sneezing or coughing distance. The sporadic opportunities for sexual transmission put sexually transmitted pathogens in a difficult situations. They have to stay viable within a person until the person has a new sexual partner .
They have to persist in the face of an immune system that is superb at recognizing and destroying foreign invaders. Then they must be transmissible to the next sexual partner when the opportunity arises. And that is just to break even. To make a profit, the pathogens must meet these challenges for at least another round of partner change the more rounds, the better their competitive advantage.
Respiratory pathogens typically reproduce to the point of contagiousness and then get wiped out or at least sequestered by the immune system within a week or two. A sexually transmitted pathogen using such a strategy would be cut out of the competition. To evade this fate, sexually transmitted pathogens must employ sneaky tricks.
Sexually transmitted diseases must act like criminals living in a town that is heavily patrolled by police. Like human criminals, they have developed a variety of strategies for evading recognition, surveillance, and capture. Because the immunological soon become very familiar with their appearance, most sexually transmitted pathogens keep a low profile.
The bacteria that cause syphilis persist by striping off many of the external molecules that would make them recognizable to the immune system. They are criminals who sand off their fingerprints.
HIV impersonates a police officer, wrapping itself in the immune cell’s own membrane when it buds off a cell. To stay ahead of the current mug shut, it frequently modifies its telltale features by mutating and recombining its genetic instructions genetically engineered plastic surgery.
The bacteria that causes gonorrhea are quick change artist, wearing different external molecules from day to day to avoid being tracked down after they are recognized. They also hide out away from the areas of most active surveillance, causing damage to the reproductive tract but not so much that it generates a sweeping immunological assault.
Herpes viruses keep a low profile by hiding out in the neurons. There they are relatively safe because an immune system that destroys neurons could irreparably cripple the body. When a person is under stress, the viruses in the neurons break their latency and begin producing progeny, which migrate down the neuronal fibers. Presumably stress is an indication that it is a good time to break out of their hiding place either to get out before something bad happens to the person, or to get out while the immune system may be less able to mount a defense.
For these pathogens, the disease is not the goal of their activities. Rather, it is the coast of doing business. These outlaw strategies seem tenuous, yet sexually transmitted diseases are very successful in human populations. They are also so successful because sex is so seductive. From an evolutionary viewpoint, sexually transmitted diseases are costly to a person, but the cost of no sex is higher. If a person were genetically programmed not to be interested in se3x, those genetic instructions would disappear in about one generation.