Scientists have struggled to understand the correlation between cold weather and the flu. This winter has seen a particularly severe flu season for a number of reasons. A wintertime spike in flu cases isn't only because of the chill outside, says Linsey C. Marr, a professor of civil and environmental engineering at Virginia Tech. It's also because of the conditions inside.

The Air Test

The link between the flu virus and air humidity has long been studied, but the results were never definitive. Last year, Dr. Marr, her doctoral...