During the last 125 years, there has been a massive decrease in mortality from
infectious disease, as a result of improved public health and later as a result of
antibiotic use. This has led to a consequent increase in life span from less than 50 (when
cancer is rare) to more than 70 (when
cancer is common). As a result of these changes,
cancer has moved from being a very minor cause of mortality in 1876 to one of the two major causes by 2000. During the last century theories of the causation of
cancer have changed progressively. In the early phase the main cause was thought to be environmental contamination with carcinogenic chemicals. Later, it was thought that all
cancers were caused by
cancers, then that
cancer was caused by a breakdown in immune-surveillance. For 40 years we have known that tobacco
smoke is a major cause of respiratory and other
cancers, and preventive measures are currently having a dramatic effect on male
lung cancer mortality. During the last 25 years it has become apparent that diet is the other major cause of
cancer, but theories have moved steadily from a search for causal agents (e.g., too much fat) to
protective agents (e.g., too little fruit and vegetable). Currently it is accepted that all of the above theories contribute to the total
cancer risk.