Dear Roach: What’s the reason for the flu season?

Keith Roach
To Your Health

Dear Dr. Roach: Why is there a flu season instead of it being a year-round health challenge? And why do flu strains originate in Southeast Asia instead of, say, Poughkeepsie, New York?

– M.S.

Dear M.S.: Why there is a flu season is a bit of a mystery. Scientists used to think it was due to people staying indoors more during cold weather or because vitamin D levels are low. But research from tropical countries suggests that it has more to do with the combination of temperature and humidity that is optimal for influenza virus transmission, which likes cooler and drier conditions. In areas with very little variation during the year, there isn’t a strong predictable flu season. There remain sporadic cases of influenza even in North America throughout the year.

Influenza can grow and cause disease in the respiratory systems of not just humans, but also pigs and some birds, such as ducks and chickens. That’s why you hear about “swine flu” and “bird flu.” In some areas of Southeast Asia, all three may live under one roof, which allows the virus to more easily “swap” genetic material and mutate into different strains.

Some of the deadliest influenza epidemics have come when genetic information from one species is transmitted to another. In 1918, for example, it seems that the virus came from birds to humans. Five hundred million people became ill and 50 million died in that epidemic worldwide. The world mostly called it the “Spanish flu,” but the first cases known were actually in the United States — not Poughkeepsie, but at an Army camp in Kansas. Still, it appears it may have started in China, after all, a year previously.

Fear of another such epidemic (”pandemic” is used for such an enormous outbreak) keeps epidemiologists — the scientists who study diseases — awake at night, and drives the search for better vaccines to stay ahead of the constantly mutating influenza virus.

Readers may email questions to