That’s counter-intuitive – we normally think that the damp makes us ill, rather than protects us from disease. But to understand why, you need to grasp the peculiar dynamics of our coughs and sneezes. Any time we splutter with a cold, we expel a mist of particles from our nose and mouths. In moist air, these particles may remain relatively large, and drop to the floor. But in dry air, they break up into smaller pieces – eventually becoming so small that they can stay aloft for hours or days. (It’s a bit like the mist you get when you turn a hose pipe to its finest spray.) The result is that in winter, you are breathing a cocktail of dead cells, mucus and viruses from anyone and everyone who has visited the room recently.
What’s more, water vapour in the air seems to be toxic to the virus itself. Perhaps by changing the acidity or salt concentration in the packet of mucus, moist air may deform the virus’s surface, meaning that it loses the weaponry that normally allows us to attack our cells. In contrast, viruses in drier air can float around and stay active for hours – until it is inhaled or ingested, and can lodge in the cells in your throat.
There are some exceptions to the general rule. Although the air on aeroplanes is generally dry, it does not seem to increase the risk of catching influenza – perhaps because the air conditioning itself filters out any germs before they have a chance to circulate. And although the dry air seems to fuel the spread of flu in the temperate regions of Europe and North America, some contradictory results suggest the germs may act somewhat differently in more tropical areas.