Why Do We Get Sick In Winter?

19 July 2016

The winter season is characterised by violent sneezes, intermittent sniffles and runny noses. As flu season makes its dutiful rounds each year, parents around the world caution their children. ‘Put on a jacket or you’ll catch a cold,’ my mother still says to me as I leave home on a chilly day.

It’s a phrase that we’ve all heard in our childhoods (some in our adulthoods too).

Semantically, this is entirely logical: ‘influenza’ is derived from the Italian ‘influenzza de freddo’, which translates to the ‘influence of being cold’ and the common ‘cold’ is pretty self-explanatory.

But how much truth is there to this old wives’ tale? Can you catch a cold from being cold?

For years, science has been arguing that the common cold and flu cannot be caused by temperature. Colds are caused by a strain of viruses – ‘rhinoviruses’ – of which there are more than 100, whilst the flu is caused by the influenza virus. Feeling a bit chilly, in the absence of these viruses, is not enough to make you sick.

However, there is some truth to mothers’ common sense – while cold may not be the primary cause, it does make us more susceptible to infection.

Ron Eccles, director of the Common Cold Centre in Cardiff, wanted to know whether cold and damp conditions activated the influenza virus. He conducted a study in which volunteers were chilled in a laboratory setting and then returned to their daily lives. Half of the participants sat with their bare feet in cold water for 20 minutes, while the other half kept their socks and shoes on and kept their feet in an empty bowl. Four or five days later, twice as many people in the cold-water group had developed a cold.

While the flu season assuredly emerges in cold and rainy conditions, there is very little understanding as to why this occurs. With this understanding, scientists could search for more effective ways to stop its pernicious spread.

Behavioural theories have some traction. In the cold wintry months, we spend more time huddled together in the toasty indoors – in other words, we’re in closer contact with potentially infectious humans.  

From a physiological point of view, the cold wears down our bodies’ natural defences. As we approach winter solstice, daylight becomes scarce and we may run low on Vitamin D levels – which at normal levels boosts our immune systems. Furthermore, when we breathe in cold air, the blood vessels in our nose constrict, blocking off white blood cells (our natural germ killers).

The soundest theory has to do with the way our sneezes linger in the air. In 2007 Dr Peter Palese, a microbiologist and an expert in the field of RNA viruses, tested the spread of flu in guinea pigs. He inoculated batches of guinea pigs with influenza virus and placed them in cages adjacent to uninfected guinea pigs. The pairs of guinea pig cages were placed at varying temperatures and humidities. Palese’s study concluded that the influenza virus spreads like wildfire in dry conditions, but struggles to gain traction in moist air. By comparing 30 years’ worth of climate records with health records, Jeffrey Shaman and his colleagues at Columbia University found that flu epidemics almost always followed a drop in humidity. According to Jane Metz, who recently reviewed all this evidence for the Journal of Infection, the overlap of the two graphs was so close that ‘you could pretty much put one on top of the other’.

A paper from the 1960s first suggested that the influenza virus has a longer survival time (the amount of time that the virus remains viable and virulent) in cold, dry air. According to the laws of thermodynamics, cold air carries less water vapour before falling as rain. So even if it is raining, although it may seem wetter outside, the air is actually drier, fostering the perfect environment for the flu virus to flourish. Every time we sputter with a cough or sneeze, we expel tiny droplets from our nose and mouths. In moist air, these particles remain large and drop to the floor. In cold, dry air, these particles break into smaller particles that can stay aloft for hours or even days. In winter, you end up breathing in a fairly unpleasant cocktail of mucus, viruses and dead cells from anyone and everyone who came into that vicinity.

So while you may want to wear one less layer than you should and ignore mum’s refrain, ‘put on a jacket!’, you should probably heed her advice. So cosy up in your thermals, sweaters and winter-wear and while you’re at it – you should probably get your flu shot too.

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