Whatever happens in the current outbreak of swine flu in the UK, the disease may not peak until this winter. In the 1957 pandemic, the flu arrived here in summer but nothing much happened until the following winter, and in the 1968-69 outbreak it took about a year to take hold.
Winter is also the time for the usual flu outbreaks, but why the season makes any difference to the disease is not well understood.
One idea is that people spend more time indoors in winter so they are more likely to infect each other. And because there is less sunshine in winter, the ultraviolet rays in sunlight are less likely to destroy the virus.
But the weather could play a more direct role. When we cough or sneeze, tiny droplets carrying the virus shoot into the air and hang around until they drop to the ground or get breathed in by someone else.
One idea is that because the flu virus is coated in a hard, fatty material it is protected in cold weather. But when the virus is inhaled its coating melts in the warm airways inside the body and infects cells.
Another reason could be that the mucus in the airways normally flows up to clear out contaminants. But mucus thickens as cold air hits the upper airways, making it more difficult to expel the virus.
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