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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

Vitamin D production in human skin occurs only when UV radiation exceeds a threshold. From simulations of UV irradiances, the VitD www page ( http://zardoz.nilu.no/~olaeng/fastrt/VitD.html) computes the daily duration of dermal vitamin D production at midday when UV radiation exceeds the required threshold. The daily duration depends on latitude, time, total ozone, clouds, aerosols, surface reflectivity and altitude all of which can be specified by the user

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

Ecological studies of ultraviolet B, vitamin D and cancer since 2000. Grant WB, Mohr SB. Ann Epidemiol. 2009 Jul;19(7):446-54. Epub 2009 Mar 9. PMID: 19269856 CONCLUSION: These findings provide strong evidence that vitamin D status plays an important role in controlling the outcome of cancer. Support for the UVB-vitamin D-cancer theory is now scientifically strong enough to warrant use of vitamin D in cancer prevention, and as a component of treatment. More research studies would help to explore whether there are benefits beyond the substantial effects that have been observed.

Ecological Studies Of Ultraviolet B, Vitamin D And Cancer Since 2000. Grant WB, Mohr SB. Ann Epidemiol. 2009 Mar 6. [Epub ahead of print] PMID: 19269856 doi:10.1016/j.annepidem.2008.12.014