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. What struck me was that both statements couldn't be true. And I asked why this was so, and the answer that was given was in a sense, 'Because.' There was no satisfactory answer.

Academy of Achievement interview (1991)