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EPIDEMICS' TIMING DETERMINED BY LATITUDEGoing back to 1945, Hope-Simpson discovered that influenza epidemics above 30 degrees latitude in both hemispheres occurred during the six months of least solar radiation and that outbreaks in the tropics almost always occured during the rainy season. He thus concluded, "Latitude alone broadly determines the timing of the epidemics in the annual cycle, a relationship that suggests a rather direct effect of some component of solar radiation acting positively or negatively upon the virus, the human host, or their interaction." That is, something may be regularly reducing our immunity every fall and winter. In 2003 researchers confirmed that influenza epidemics in the tropics occur, with few exceptions, during the rainy season, when vitamin D levels should be falling

Addressing the health benefits and risks, involving vitamin D or skin cancer, of increased sun exposure.\nMoan J, Porojnicu AC, Dahlback A, Setlow RB.\nProc Natl Acad Sci U S A. 2008 Jan 15;105(2):668-73. Epub 2008 Jan 7.\nPMID: 18180454