Regions of the world subjected to heavy infectious burdens seem to show lower incidence of
cancer. The index of infectious burden in this study has been chosen to be poverty, i.e., low
GDP, high infantile mortality, poor hygienic conditions, inaccessibility to modern medical infra-structures, etc. When estimates of observed
cancer incidence is plotted against the
GDP of 24 regions of the world, a trend line is obtained: low
GDP (a proxy for high infectious burden) tends to be associated with low incidence of
cancer whereas high
GDP values herald higher
cancer occurrences. Similarly, countries with high infantile mortality rates tend to have a lower incidence of
cancer and vice versa. The data are explained in terms of the so-called "hygiene hypothesis": frequent infectious onslaughts, especially in childhood, challenge the immune system and build a strong adaptive immune system and immunological memory which prepare the body to tackle further battles down the line, such as
cancer. Within this framework, the role of other factors such as diet,
selenium, hardness of water, etc. in the aetiology of
cancer is also briefly examined. For rigorous verification of this observation, age-adjusted
cancer incidence rates for various countries must be used even though such data are not available for all the countries examined here [Bulletin of World Health Organization 62(2) (1984) 163]; where data are available [Age-adjusted death rates for
cancer for selected sites (A classification) in 51 countries in 1974, Segi Institute of
Cancer Epidemiology, Nagoya, Japan (Feb. 1979); Global Geocancerology, Churchill Livingstone, New York, 1986], the same trend is also observed for the age-adjusted
cancer death rates, which may be used as an index of age-adjusted
cancer incidence, subject to qualifications discussed in the text.