SINCE the dramatic decline in the mortality of infectious diseases, increasing attention has been focused on degenerative and neoplastic diseases as the major causes of death. In considering causation of degenerative and neoplastic conditions the need for definite proof is of paramount importance. For the purpose of brevity, these noninfectious diseases will be referred to as chronic diseases, though it is apparent that some infectious diseases may also be chronic, and some neoplastic diseases may have an acute course.
In contrast to infectious diseases, chronic diseases are not likely to be due to one specific factor, but rather are a result of multiple factors usually operating over a long period of time. Bacterial diseases, of course, also may have multiple causes, but there is one necessary etiologic agent. There is no evidence, however, as yet that this type of relationship applies to any of the chronic diseases studied. One must,