In the summertime, in the middle of a heat wave, you're not thinking "flu season." Yet, if you recall, last summer we were in midst of one.

I remember a lot of people getting sick from the H1N1 flu virus. It challenged the usual pattern of the flu which normally infects one percent of Americans in August compared to January when that number nearly doubles.

That's seven and a half million people!

Why are more people sick in the colder months? Is your grandmother right? Is it because you aren't bundling up? For a long time we didn't know the answer, but recent studies give us some clues.

The two key components are temperature and relative humidity. In the lab, researchers looked at how temperature and humidity affected transmission of the flu from infected to uninfected animals.

What they saw is that, at room temperature, or 68 degrees Fahrenheit, influenza was transmitted the best at 50 percent humidity or less. But when they raised the humidity to eighty percent, there was no transmission of the virus!

The highest rate of transmission occurred when they lowered the temperature to a chilly 41 degrees, with a humidity of 50 percent or less. But just by turning up the temperature to 86 degrees, no virus was transmitted.

So this means the flu virus transmits best at both a low temperature and a low relative humidity. Was granny right? Is it because resistance to viruses is altered in the cold? The answer is no.

In cold conditions, the infected animals shed higher levels of the virus. And influenza also appears to survive better in cold air with low humidity.

So now we know bundling up doesn't work. That makes washing your hands often even better advice.