Scientists have long debated why flu erupts when the days grow chilly. Is it that we spent more time cooped up together indoors? Or is there something about the virus likes it cold and dry? To find out, researchers suspended influenza virus in a solution that mimics human mucus. They incubated this infectious soup at different huminities and measured viral survival. 

   

  And they found that at low huminity, the fake mucus dries up and the virus does just fine. But when the humidity tops 15 percent, the droplets only partially evaporate, leaving behind a solution that’s too salty for the virus to thrive. 

   

  Interestingly, the virus does well again when the humidity reaches 100 percent. Evaporation stops and this humidity of the mucus bath is just right. That could explain why the flu prefers to hit the tropics in rainy season and why you should always keep your nose clean, but moist.