Why is there a flu season? It’s a surprisingly difficult question to answer. In most cases, the traditional and somewhat superstitious equations between viral infections and weather turn out not to hold water – for example, you catch cold from being exposed to the cold virus, not from being caught out in the cold and damp. Nevertheless, there is a definable and predictable flu season: specifically, when the weather turns cold in the late fall and over the winter. Why this is so is surprisingly difficult to explain.
The world actually has two such flu seasons, because winter occurs at opposing times of the year in the northern and southern hemispheres. The precise timing varies somewhat in every year, but in general the peak of the flu season lasts between six weeks and two months. This period occurs in North America, roughly, some time between October and the following May.
Because the influenza virus does not mutate as quickly or as unpredictably as the common cold, it is possible to develop vaccines for particular strains. Over the winter, the number of flu victims often increases by an order of magnitude in the general population, and a large percentage of new cases tend to suffer from a single major flu type (the swine flu of 2009, for example was of the flu subtype H1N1). For this reason, pharmaceutical companies attempt to predict the most likely viral recombinations over the next year, and then construct an annual flu vaccine based on these educated guesses.
Surprisingly, while there is general agreement about the existence of the flu season (which is obvious to any observer), there is still considerable debate about why the flu season actually happens at all. It is possible that the old saw about getting sick in cold weather actually has some truth to it: while exposure to the influenza virus is the only necessary factor in catching the flu, it is possible that dry, cold weather makes the body more vulnerable to respiratory infections like the flu by reducing the amount of mucus in the body’s respiratory system (mucus catches foreign particles, including viruses, and then expels them from inside the body).
Another climate-related possibility is that reduced sunlight in the winter months results in reduced Vitamin D production, which in turn could cause reduced immune system effectiveness. It’s even possible that the influenza virus has evolved some specific biological traits which allow it to function more effectively in cold weather than warm weather; if a virus copy can survive outside the body longer in cold weather than in warm weather, for example, that would effectively make the virus more contagious in winter than in summer.
The second category of explanations are social rather than environmental. Cold weather causes changes in how we relate to each other. For example, people stay indoors for longer periods of time, in closer contact with others. This is true in general, but is most especially true of schools, which break for the summer months. When large numbers of people spend longer amounts of time in close contact with each other, it becomes far easier for viruses to spread throughout the population. When the weather warms up and people move outside again, infection rates drop.
Further study of the flu virus will doubtless help provide a more confident explanation. For now, the best that can be understood is that the flu spreads more quickly throughout the population in cold winter weather – we’re just not sure why.