However, his most enduring legacy is the observation that nuns had more breast cancer and less cervical cancer when compared to the general population of females living in Padua (his hometown). It was a puzzling observation, and one that he did not expect (at the time, one theory about breast cancer was that it resulted from "vigorous sexual concourse"). His explanation followed a similar line of reasoning - the breast is a sexual organ, and without regular sexual activity it would decay. Cancer was the result. Interestingly, he did not apply this same logic to the low incidence of cervical cancer in nuns, but he did propose that it was somehow related to the celibate lifestyle.
With cervical cancer, the link is even more dramatic - almost all cases of cervical cancer (>99%) are caused by human papillomavirus (HPV), a sexually transmitted disease. Since the nuns were celibate, they never acquired the virus and therefore never developed cervical cancer. Taken together, these two observations were the first inkling that modern science into lifestyle factors as a possible cause of cancer. We still use this thinking today - smoking is a lifestyle factor, and its role in the development of lung cancer is widely accepted as fact. So although Ramazzini was right for the wrong reasons, his investigation of breast cancer in nuns is one of the great early epidemiologic studies and demonstrates how epidemiology can help us to understand ways to prevent disease and puzzle out risk factors even without a firm grasp of the biologic cause.