An epidemiologic approach to host factors in the etiology of cancer

We examined the contribution of host factors in carcinogenesis by evaluating the frequency distributions and secular trends in available epidemiologic data of mortality and morbidity around the world. In most developed countries, the age-adjusted death rates of cancer of all sites have levelled off in recent years and the intercountry difference in mortality is small. Deaths from cancer account for approximately 20% of total deaths in these countries. It is believed that the difference in cancer mortality of all sites between the developed and developing countries can be explained by the different levels of mortality from infectious and parasitic diseases, including deaths from tuberculosis. A certain upper limit for the number of cancer deaths in each population is strongly suggested by the unaltered cumulative mortality rate from cancer and tuberculosis up to 85 years of age in a defined birth cohort in the developed countries. The apparent age dependency in cancer mortality as well as some other essential epidemiologic implications in carcinogenesis, the commonly observed proportion of cancer deaths of around 20% of total deaths, and the well-known etiologic importance of environmental factors which determines the site-specific cancer mortality and morbidity lead one to consider the definite existence of "susceptibles to cancer" in each population. However, the concept of susceptibles to cancer is different from the usual one of hereditary predisposition to so-called "intractable diseases," which have low incidence rates.