Most of the carcinogenic factors that are known to be significant are by no means specific to the modern world. The most potent known carcinogen, by certain assays at least, is aflatoxin B1 (see Figure 20–38). It is produced by fungi that naturally contaminate foods such as tropical peanuts and is an important cause of liver cancer in Africa and Asia.
Except for tobacco, chemical toxins and mutagens are of lesser importance as contributory causes of cancer than other factors that are more a matter of personal choice. One important factor is the quantity of food we eat: as mentioned earlier, the risk of cancer is greatly increased in people who are obese. In fact, it is estimated that as many as 50% of all cancers could be avoided by simple, identifiable changes in lifestyle (see Figure 20–37B).
Viruses and Other Infections Contribute to a Significant Proportion of human Cancers
Cancer in humans is not an infectious disease, and most human cancers do not have any infectious cause. However, a small but significant proportion of human cancers, perhaps 15% in the world as a whole, are thought to arise by mechanisms that involve viruses, bacteria, or parasites. Evidence for their involvement comes partly from the detection of viruses in cancer patients and partly from epidemiology. Thus, cancer of the uterine cervix is associated with infection with a papillomavirus, while liver cancer is very common in parts of the world (Africa and Southeast Asia) where hepatitis-B viral infections are common. Chronic infection  VINYL CHLORIDE:  BENZENE:  ARSENIC: skin carcinomas, bladder cancer  ASBESTOS:  RADIUM:
Figure 20–39 Age-adjusted cancer death rates, United States, 1930–2008.
Selected death rates, adjusted to the age distribution of the US population, are plotted for (A) females and (B) males. Note the dramatic rise in lung cancer for both sexes, following the pattern of tobacco smoking, and the fall in deaths from stomach cancer, thought to be related to a fall in rates of infection with
Helicobacter pylori. Recent reductions in deaths per 100,000 populationdeaths per 100,000 population0 20 40 60 80 0 20 40 60 80stomach breast lung and bronchus uterus ovary pancreas stomach prostate lung and bronchus leukemia liver pancreas treatment. Age-adjusted data like these are needed to compensate for the inevitable increase in cancer as people live longer, on average. (Adapted from Cancer Facts and Figures, 2012. Data from U.S. Mortality Volumes 1930 to 1959, U.S. Mortality Data 1960 to 2008, National Center for health Statistics, Centers for Disease Control and
Prevention. © 2012, American Cancer (A) (B) Society, Inc., Surveillance Research.) with hepatitis-C virus, which has infected 170 million people worldwide, is also clearly associated with the development of liver cancer.