The real reason germs spread in the winter

The flu season arrives so predictably, and affects so many of us, that it’s hard to believe that scientists have had very little idea why cold weather helps germs to spread. Over the last five years, however, they have finally come up with an answer that might just offer a way to stem the tide of infection – and it revolves around a rather grim fact about the ways that your sneezes linger in the air. 

A new understanding of influenza couldn’t come quickly enough; worldwide, up to five million people catch the illness each flu season, and around a quarter of a million die from it. Part of its potency comes from the fact that the virus changes so quickly that the body is rarely prepared for the next season’s strain. "The antibodies we’ve built up no longer recognise the virus – so we lose our immunity,” says Jane Metz at the University of Bristol. It also makes it harder to develop effective vaccines, and although you can engineer a new jab for each strain, governments often fail to persuade enough people to take it up.

The hope is that by understanding better why flu spreads in winter, but naturally fades in summer, doctors could find simple measures to stop its spread. Previous theories had centred on our behaviour. We spend more time indoors in the winter, meaning that we’re in closer contact with other people who may be carrying germs. We’re more likely to take public transport, for instance – and as we’re pressed against spluttering commuters, misting up the windows with their coughs and sneezes, it’s easy to see how this could send us over a tipping point that allows flu to spread through a population.

Another popular idea concerned our physiology: the cold weather wears down your body’s defences against infection. In the short days of winter, without much sunlight, we may run low on Vitamin D, which helps power the body’s immune system, making us more vulnerable to infection. What’s more, when we breathe in cold air, the blood vessels in our nose may constrict to stop us losing heat. This may prevent white blood cells (the warriors that fight germs) from reaching our mucus membranes and killing any viruses that we inhale, allowing them to slip past our defences unnoticed. (It could be for this reason that we tend to catch a cold if we go outside with wet hair.)

While such factors will both play some role in transmission, analyses suggested that they couldn’t completely explain the yearly emergence of flu season. Instead, the answer may have been lying invisible in the air that we breathe. Thanks to the laws of thermodynamics, cold air can carry less water vapour before it reaches the "dew point” and falls as rain. So while the weather outside may seem wetter, the air itself is drier as it loses the moisture. And a steady stream of research over the past few years has shown that these dry conditions seem to offer the perfect environment for the flu virus to flourish.

(BBC)