Outbreaks of influenza typically occur in the winter, when the environment features colder air with a low water content, or low humidity. In fact, air in the winter can have as much as four times less moisture than air in the summer. And the flu likes dry air.

This could be another explanation of the seasonal nature of flu. In this case, researchers are talking about absolute humidity, not relative humidity, the more common reading. Relative humidity is a ratio not an absolute amount. Absolute humidity, on the other hand, is the actual amount of water in a volume of air. And it is the amount of water that concerns the virus. It seems that the water in the air affects the virus itself, although the exact mechanism is not clearly understood.

What is clear is the discovery that when the absolute humidity is low, that is to say the air is dryer; the survival rate of the virus is greater. Longer survival equals more transmission, meaning higher infection rates.

Once the numbers were analyzed, it was seen that more flu cases were discovered when it was both colder and drier. In temperate regions of North America and Europe, absolute humidity has a powerful cycle that is seasonal. These changes, which dramatically lessen in the wintertime, parallel an increase in the rates of transmission of the virus as well as the rate of survival of that same influenza virus.

By using absolute humidity in this way, an additional factor in the spread of influenza and the endurance of the virus can be an added to the creation of models of infection, or models for the prediction of viral spread. Doctors Shaman and Kohn, authors of a study published in the Proceeds of the National Academy of Science on March 3, 2009, were quick to point out that this is a preliminary study and further exploration of this phenomenon is needed, especially in the areas of epidemiology, modeling and lab work into the actual structure of the virus.

Still, this remains an important potential insight into the way the flu virus is spread, and consequently how to keep the flu virus from spreading. Until all mechanisms of flu infection are better understood, the flu vaccine remains the best protection against becoming infected.