Certain things happen in syphilis that are explained by most experts in terms of immunity, although others, accepting the facts, explain them in different terms and argue that there is no true immunity to this disease. The argument has been going on since the prebacteriologic era and has never been completely resolved. It has generated an enormous literature and a lot of ingenious research. If we ever do have a syphilis vaccine it will have grown out of this argument. But this is not the place for the details: let me try to give you the essentials as simply as I can.

In untreated syphilis, or in syphilis that is not treated until several months or more have elapsed after the first appearance of symptoms, another exposure usually does not renew the disease. In the untreated syphilitic this is obviously not immunity, since he is still infected. When he is treated late in the disease and cured, he seems to be immune; but as we saw previously, there is a residual doubt as to whether the cure is complete.

The same picture is seen in experimental syphilis in rabbits, and has in fact been confirmed in experiments on human volunteers. In both man and rabbit, when the disease is cured in the early infectious stage, reinfection can take place with a new chancre. Repeated reinfections were first seen in 1943, soon after the introduction of penicillin, especially when only one sex partner was treated. The result, as I have mentioned earlier, was given the name "ping pong" syphilis. At that time the occurrence of such new chancres, or reinfections, was accepted as evidence of complete cure, something which had otherwise been thought by many to be impossible. In short, everyone was agreed that reinfection did not happen while the old infection was present. The controversy centered on the question, did the absence of reinfection after late cure mean immunity, or did it mean persistence of infection, even though the infection might prove permanently latent?

What resolved this question for most people, in favor of a true immunity, was the demonstration of antibodies against the spirochetes, culminating in the TPI test. These antibodies abolish the virulence of the spirochetes even before they stop their movements. They seem, accordingly, to be a factor in immunity to syphilis, although they are evidently not the only one. But the fact that they actually kill the spirochetes is a focal point of hope for an eventual vaccine.

Syphilis is unusual in that its immunity if we assume there is such a thing develops very slowly and is not directly measurable in terms of antibodies. But if the fact of immunity be accepted, the search for a vaccine becomes the more promising.