The answer: Vitamin D deficiency

The questions:

Why is the flu primarily a "wintertime" illness if the influenza virus exists in the population year-round?

Why does the flu disappear in the months following the summer solstice?

Why is influenza more common in the tropics during the rainy season?

Why is the incidence of influenza inversely correlated with outdoor temperatures?

Why are children exposed to sunlight less likely to get colds?

Why does cod liver oil reduce the incidence of viral respiratory infections?

Why is it that volunteers, deliberately infected with a weakened flu virus - first in the summer and then again in the winter - show significantly different clinical courses in the different seasons?

Why are Blacks more likely to die from influenza and pneumonia than Whites are?