Science Questions

There are a number of factors.
You can only catch the virus from other people (practically speaking) and in Winter lots of people tend to get more crowded together in nice warm buildings. Also the ventillation is usually less vigorous because they want to keep the heat in.
Drying out or ultraviolet light from the sun will inactivate the virus; you can see how it survives longer in Winter when it's darker and wetter.
There's also an effect that people who are cold, wet and miserable, have reduced immunity to illness.

I think its because most people's immune system to prone to various pathogens. In the winter, people tend to gain more pounds and eat heavier. Unlike in the summer, people are more active and eat less. Also, the cold weather does have an effect on viral infections as well, where it is easily spread

Epidemic influenza and vitamin D.

Cannell JJ, Vieth R, Umhau JC, Holick MF, Grant WB, Madronich S, Garland CF, Giovannucci E.
Atascadero State Hospital, 10333 El Camino Real, Atascadero, CA 93422, USA. jcannell@dmhash.state.ca.us

In 1981, R. Edgar Hope-Simpson proposed that a 'seasonal stimulus' intimately associated with solar radiation explained the remarkable seasonality of epidemic influenza. Solar radiation triggers robust seasonal vitamin D production in the skin; vitamin D deficiency is common in the winter, and activated vitamin D, 1,25(OH)2D, a steroid hormone, has profound effects on human immunity. 1,25(OH)2D acts as an immune system modulator, preventing excessive expression of inflammatory cytokines and increasing the 'oxidative burst' potential of macrophages. Perhaps most importantly, it dramatically stimulates the expression of potent anti-microbial peptides, which exist in neutrophils, monocytes, natural killer cells, and in epithelial cells lining the respiratory tract where they play a major role in protecting the lung from infection. Volunteers inoculated with live attenuated influenza virus are more likely to develop fever and serological evidence of an immune response in the winter. Vitamin D deficiency predisposes children to respiratory infections. Ultraviolet radiation (either from artificial sources or from sunlight) reduces the incidence of viral respiratory infections, as does cod liver oil (which contains vitamin D). An interventional study showed that vitamin D reduces the incidence of respiratory infections in children. We conclude that vitamin D, or lack of it, may be Hope-Simpson's 'seasonal stimulus'.

Epidemiol Infect. 2006 Dec;134(6):1129-40. Epub 2006 Sep 7.



...from the final conclusion in the full-text:

  Today, in a rush from multiplex reverse transcriptase-polymerase chain reactions that rapidly subtype influenza viruses to complex mathematical formulas that explain infectivity, many of us have forgotten Hope-Simpson's simple 'seasonal stimulus' theory for the lethal crop of influenza that sprouts around the winter solstice.  The faith and humility that characterized his life and his writings insulated him from despairing that his 'seasonal stimulus' would not be sought.  Among his last published words was the suggestion that 'it might be rewarding if persons, who are in a position to do so, will look more closely at the operative mechanisms that are causing such seasonal behaviour' .

Dr Edgar Hope-Simpson  (1908-2003)
       
http://www.makingthemodernworld.org.uk/learning_modules/geography/05.TU.01/?section=6
http://www.astrobiology.cf.ac.uk/image15.gif

A Gloucestershire GP carefully recorded the incidence of influenza in his practice over a period of nearly 30 years. Dr Hope-Simpson obtained a picture of the timing and intensity of these cases from 1946 to 1974.
Is it possible to compare Kilbourne’s chronological model of the spread of influenza with this data?
Such a comparison indicates that there should be evidence of the following factors influencing the final picture:
- A distinct seasonal pattern, with the highest incidence in winter.
- A series of decreases in the size of epidemic waves as the population becomes immune to one particular strain of the virus.
- The appearance of a new strain with changed antigens, meaning that the body’s defence mechanism does not recognise it. The whole process of infection should then begin again.
- The presence of more than one strain of influenza in the population at any one time.
- Newly introduced strains from other parts of the world, which can be especially virulent.

for more reading click here:  http://www.makingthemodernworld.org.uk/learning_modules/geography/05.TU.01/?section=6

Lower humidity in winter means drier membranes in the nasal passages and throat.  In the former at least, this commonly causes cracking and bleeding (more bloody discharge in winter for sure), which would seem to afford easier direct entry points into the blood stream for virii.  And simply drier membranes means smaller positive flow (outward) of protective mucus current, and this could give passage for virii to diffuse inward.
 
One way or another, the virii are most certainly getting in via the respiratory tract.  They don't pass through the skin between the toes.  It would seem that the humidity factor should not affect the state of the digestive tract, so the degree to which virii infect from there, if at all, wouldn't change seasonally.  Deduce from that very little to no flu infection via the digestive tract, since flu is much less common in warmer weather.

Once a person catches a bug, the overcompensation of copius amounts of mucus is too late.  But if the humidity factor is really important, a preventative treatment that induces sufficient, constant, mucus from the outset could help, like a regular dose of time-release guaifenesin.  Of course environmental humidification wherever possible.

Absolutely its the falling levels of vitamin D3 that makes people so susceptible to flu and colds in the winter.
Most people are deficient in D3 anyway and when winter comes they have little defense at all.

I keep my levels above 60. I never get sick. It also cured my psoriasis when I brought my levels up to 130 for a while