MINNEAPOLIS (WCCO) — Minnesota’s official influenza season started in October and will last through April.

It’s the same for much of the northern hemisphere. In the southern hemisphere, the months are reversed. Flu season down happens during their winter – from May through September.

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Across the globe, there is rarely a flu case during the summer.

So, that had Beth from New Ulm wondering: Why do we only see the flu in the winter?

“There’s a lot of controversy around this,” said Allina Medical Clinic Infectious Disease expert Dr. Frank Rhame.

He credits much of the reason to crowding.

“When winter comes, we get closer together, the windows get closed, the ventilation isn’t as good, so airborne spread is more likely to occur,” he said.

And why there is winter flu in warmer states like Florida, where much of their crowding occurs in the air-conditioned summers, Rhame said the spread of influenza has a “whole country effect” that requires a critical mass.

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“Even though Florida is more temperate than we are, within the country there’s a ramping up of people getting closer together, and they have to give it to each other to get it in full force,” he said.

And, as Chris Cloud of Minneapolis – who came down with the flu last December — pointed out, “There’s these things called planes that people get on from all over the world.”

Kris Ehresmann, head of infectious diseases at the Minnesota Department of Health, offers a different theory.

“It’s likely related to humidity,” she said.

In 2007, Mount Sinai hospital flu researcher Dr. Peter Palese wrote a study that found dry conditions favor transmission of the virus.

“Our mucous membranes get drier in the winter and they may be more susceptible to infection when they’re drier,” Rhame said.

Other researchers have suggested less exposure to Vitamin D during the winter might also contribute to the spread of the flu in the winter.

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In the summer of 2009, the H1N1 strain of the flu virus started surprised scientists with its unique combination of genes they’d never seen before. Researchers said it’s still unclear why that strain was so powerful in the summer, but point out that after its first 18 months, it transitioned into a winter virus and stayed there ever since.

Heather Brown