Whether we live in the southern hemisphere or the northern hemisphere, the flu season occurs in the winter months. In the tropics there is no defined flu season and, in fact, there is not much influenza at all. So why do we get flu in the winter? One popular hypothesis suggested that in the winter we come in contact with each other much more as we tend to stay indoors in bad weather. But that seems unlikely since, nowadays, we are often crowed together on trains, at school and in offices all year around. Another suggestion was the lack of light during the winter: Perhaps the immune system does not work as well under low light conditions; maybe we do not make enough melatonin for a well-primed immune system or perhaps it is vitamin D that we are lacking since this vitamin is made in the light.

In winter the climate is cold and dry whereas in summer it is warm with higher humidity. It appears that the reason for the predominance of flu in the colder drier parts of the year is a property of the virus itself rather than our seasonal environment or alterations in our innate immune system.

In 2007, it was shown using guinea pigs that at 5 degrees C (41F), the virus spreads from one animal to another rapidly but when the temperature rises to 35 degrees (95F), the virus does not spread at all. Virus release from infected animals went on for forty hours longer at 5 degrees than at room temperature (20 degrees C) which may account for the increased spread at low temperatures. Humidity is also a factor with good spread at a relative humidity of 20% and no spread at 80%.  

It turns out that influenza viruses are simply more stable under cool temperature conditions. In addition, low humidity helps spread because flu viruses spread in small water droplets (sneezes rather than person-to-person direct contact) and under high humidity these droplets become bigger and drop to the ground.

It was found that innate immunity was similar in animals housed at 5 degrees C and 20 degrees C. This implies that that cool temperature (5 degrees) does not impair the innate immune response of the guinea pigs.  

See: Lowen AC, Mubareka S, Steel J, Palese P.   

Influenza virus transmission is dependent on relative humidity and temperature. PLoS Pathog. 2007 Oct 19;3(10):1470-6.