Why do we mostly get ill with flus and colds in winter?

Dr. Liu and colleagues at UCLA, publishing in this March's edition of the prestigious journal Science, showed that vitamin D might be, in effect, a potent antibiotic. Vitamin D increases the body's production of naturally occurring antibiotics: antimicrobial peptides. Antimicrobial peptides are produced in numerous cells in the human body where they directly and rapidly destroy the cell walls of viruses and bacteria, including tuberculosis. Furthermore, Liu showed that adding vitamin D to African American serum (African Americans have higher rates of TB) dramatically increased production of these naturally occurring antibiotics.
Science. 2006 Mar 24;311(5768):1770-3.

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Gary Moller comments:

Why is it that dark-skinned populations tend to be less healthy when:

• They shift further North or South?
• They move into cities?

If we get colds and flu due to getting cold, then why is it that people who work the skifields, including pro skiers always seem to be very healthy?

Why is it that flus and colds are worst during winter, despite the fact that these bugs are present in populations year round?

Why is it that flu outbreaks in the tropics are worst during the rainy season?

The answer may be due to my pet topic for some years now: Vitmain D deficiency which is present in populations and worst during winter. We are learning all the time just why the body produces massive amounts of this vitamin when the skin is exposed to sunlight. The latest finding is that it may exert a powerful antibiotic effect on both bacteria and viruses.

When people move North or South and into cities, they tend to get much less year-round sunlight and dark-skinned people are poorly adapted for low sunlight. During the rainy season in the tropics, there may be no sunlight for weeks on end. This is exacerbated nowadays in many lands by air pollution that blocks UV. People who work on skifields get constant exposure to high levels of UV radiation, thus producing vitamin D despite the cold.

Commonsense tells us that the human body must be making Vitamin D for very good reasons. The list of good reasons grows longer by the day.