RENEE MONTAGNE, host:

It's MORNING EDITION from NPR News. Good morning. I'm Renee Montagne. In North America, swine flu hasn't spread explosively. And so far, at least, it hasn't caused a lot of serious illness. But in the Southern Hemisphere, winter is coming, which means the flu season is just beginning. So scientists will be watching to see how the new virus behaves there. NPR's Richard Knox reports on why influenza appears so reliably every winter.

RICHARD KNOX: It's autumn in Rio de Janeiro, and Dr. Evelyn Eisenstein, a Brazilian pediatrician, says the first cases of seasonal flu are starting to show up.

Dr. EVELYN EISENSTEIN (Pediatrician): It worries me so much, because children go to the school even when they have a cold, you know? Parents don't stay with them. So we have still to do lots of prevention and health education.

KNOX: Eisenstein has special reason to worry this year. She has no way of knowing whether her patients will get sick with the new flu virus she's been hearing about up north - or if they do, how sick they'll get.

So far, few cases of the new flu have shown up in Central and South America. But Dr. Jon Andrus of the Pan American Health Organization expects many, many more as the southern winter takes hold.

Dr. JON ANDRUS (Pan American Health Organization): The winter months provide the ecological situation where the virus more readily infects people. Transmission is facilitated by the conditions of winter, the grouping of people, the hunkering down due to the weather conditions, which will provide more exposure and contact. We're concerned about that.

KNOX: That makes sense, but Dr. Peter Palese, of the Mount Sinai School of Medicine in New York, says conventional wisdom doesn't really explain why flu comes around every winter.

Dr. PETER PALESE (Mount Sinai School of Medicine): Some of the theories about the seasonality over the years have sort of postulated that it's crowding of schools during the winter, that in the summertime schools are off. But that really turns out to be not the determining factor.

KNOX: Palese and his colleagues have studied how flu viruses spread among guinea pigs.

Dr. PALESE: And what we found was that specifically cold temperature, 40 degrees — exactly conditions which we have in temperate zones in the wintertime — are much more favorable for transmission. At high temperature, like 75, 80 degrees, we don't see any transmission in the system.

KNOX: At low temperatures, the virus-laden droplets that guinea pigs and people sneeze or cough out when they've got the flu shrink in size. These smaller droplets carry much further, and stay suspended in the air longer. Humidity is also a factor. Soggy, warm air is bad news if you're a flu virus.

Dr. PALESE: At higher humidity, these droplets become much bigger and actually sink to the floor, and that also contributes to basically the elimination of the virus in the aerosol.

KNOX: And colder, drier air affects the respiratory tract in a way that gives flu viruses a boost. It thickens mucus. That makes it harder for the body to clear out whatever viruses get inhaled. But it's well-known that schools are fertile breeding grounds for flu. And classrooms aren't kept at 40 degrees. I asked Palese how he squares that with his findings.

Dr. PALESE: It's more likely that actually the virus is transmitted in the hallways, which are a little bit colder; maybe outside of the school when people are waiting; in the subway, where it is colder. So it may not be actually what we think where people aggregate, namely in movie houses or in a lecture hall, that that's actually where the greatest transmission occurs.

KNOX: Other scientists think people's resistance to flu goes down in winter. One controversial hypothesis is that people don't get exposed to enough sunshine in winter. Sun stimulates the production of vitamin D, which has a role in immunity.

Whatever the factors, this winter will be an especially tense time in the Southern Hemisphere. It will reveal if the new flu virus is going to touch off the next serious pandemic, or fade away like the swine flu outbreak of 1976.

Richard Knox, NPR News.

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