MYSTERIOUS ORIGINS PART I

THE SEASONALITY OF FLU

Influenza and cold weather go hand in hand in temperate zones: In the Northern Hemisphere the flu season typically begins in November and runs to April. But flu viruses themselves circulate year-round. They show little seasonality in the tropics, and new strains can emerge during the warmer months, as the H1N1 swine flu did this year.

Researchers have come up with many possible reasons to explain why nonpandemic flu viruses take hold only at certain times of the year. Cool, dry air seems to help the virus survive on surfaces. Lack of vitamin D, the sunshine vitamin, may leave the immune systems of people weaker during winter, perhaps paving the way for infections. In chilly weather, too, people tend to stay indoors and crowd together in schools and offices, boosting the odds of person-to-person transmission. Indoor heating systems could also play a role, transporting coughed aerosols to distant areas of buildings.

The reasons sound plausible, but little research has actually gone into supporting or rejecting the theories. In a 2007 review on influenza seasonality, Eric Lofgren of the Tufts University School of Medicine and his colleagues wrote that seasonality is likely to be the result of "less-than-straightforward interaction of many different factors."