We were told in one lecture that it was possible to immunize against diphtheria and tetanus by the use of chemically-treated toxins, or toxoids. And the following lecture, we were told that for immunization against a virus disease, you have to experience the infection, and that you could not induce immunity with the so-called “killed” or inactivated, chemically-treated virus preparation. Well, somehow, that struck me. The way it struck me was that both statements couldn’t be true. I asked why this was so, and the answer that was given was, in a sense, “Because.” There was no satisfactory answer.