In 1911 Peyton Rous, working at the Hospital here at Rockefeller, discovered a virus in chickens that caused cancer, and in 1934 Rockefeller's Richard Shope discovered another virus that could cause cancer in rabbits. That means it's transmitted from one kitten to another in a litter so the whole litter gets leukemia all at once. But cancer is not usually an infectious disease in humans. So in 1961 we knew there was some disconnect between what we had gotten from the grandfathers of science and what we knew about human cancer. You can do the experiment of just plotting up age versus the incidence of cancer, the rate at which people get cancer.
Cancer is by and large a disease of the elderly.
Well, Lewis Thomas had a hypothesis. He said that the rate at which you get tumors is the same over your lifetime, whether you are one year old or 70 years old. And the reason for the curve is that when you are young, the immune system takes care of you, and it kills off all the tumors. But the immune system decays as we get old. That's not a bad hypothesis.