I wonder if people living in sunnier latitudes (lighter skinned people, that is) get sick less? Of course, there are so many variables - if they go out in the sun or not, for instance.
The variables present in attempting to study this boggle the mind: is there a correlation between sun sensitivity and Vitamin D production? (aside from the amount of melanin present)
Are those who spend less time in the sun also spending more time in enclosed environments where they are more likely to be exposed to pathogens?
Is the reason the 'summer cold' is less common than the winter one--or is that otherwise weather related (dry air spreads viruses better than moist air, because the viruses live longer--most heating systems dry the air).
At this latitude, we have substantially less sunlight per diem than in the summer, so I think I'll get a few of the clan to try the vitamin D and see if we stay healthier than the rest of the extended family.
With a pool of about 20 'subjects' the results may not be conclusive from a scientific standpoint, but if it works, it works.