In United States, the influenza virus spreads roughly from October to April, according to the U.S. Centers for Disease Control and Prevention. In Australia, the flu season is more like May to October — the Australian winter.
In fact, everywhere that experiences a winter, there’s a flu season.
So what’s the connection?
It’s not because being cold gives you a cold, as your grandmother might have you believe. There are several existing hypotheses, though why flu flourishes in the winter is “a fascinating question that’s engaged infectious disease doctors and virologists for years,” Dr. William Schaffner, the immediate past-president of the National Foundation of Infectious Diseases and professor of preventive medicine at the Vanderbilt University School of Medicine, told weather.com.
The top theory: There's something specific about the influenza virus that enables it to flourish in cold, dry weather, Dr. Schaffner said.
“Cooler air and lower humidity promote the transmission of the influenza virus from one person to another,” he said.
A study from the National Institute of Child Health and Human Development (NICHD) found that the virus forms a special outer coating in cold weather that helps it fly through the air — and into another person. Flu is an airborne illness that can be transmitted to another person from as far as six feet, according to the NIH. This cold-weather coating simply helps along this process.
“In the summer, the wet, humid air pulls the virus to the ground, so it’s less likely to be inhaled by someone else,” Dr. Schaffner continued.
Dr. Schaffner said he believes that a second theory about the flu’s winter spread — the “enclosed space theory” — is also a factor. This hypothesis holds that because individuals spend more time indoors in the winter, they’re more likely to spread it to each other. Children are particular flu spreaders in this case, he said. “Children are the great disseminators of viruses in our communities,” Dr. Schaffner said. “Children generally have closer physical relationships to each other and of course, they’re less hygienic.”
Another theory: The population’s lower levels of vitamin D in the winter (because of less sun exposure) weaken the immune system. “This would not affect the transmissibility [of the virus], but rather the likelihood that the virus would make you sick if you encountered it,” he said, adding that he’s not sure that vitamin D levels go down so substantially in the winter that they would have such a noteworthy impact.
The “great lesson” from all of this, according to Dr. Schaffner? Flu season will happen, year after year, so get an annual flu shot. The flu vaccine is more important than ever in years like this, when the H1N1 “swine” flu is running rampant, targeting disproportionate numbers of young, healthy adults.