Diseases, like the cold or flu, are caused by viruses, and some others are caused by bacteria, along with a few other causes like prions. Temperature, in and of itself, does not cause illness. To get sick, these viruses or bacteria need to get the better of your immune system.
So what’s behind the old bit of advice to bundle up? Even if the cold cannot cause illness, it can make you more vulnerable to getting sick. That’s because your immune system is more susceptible to illness because cold conditions can weaken it.
You’re Spending More Time Inside
As the colder months come around, people spend more time indoors. It is cold, after all. This means you spend more time in confined spaces around other people. That allows far more chances for diseases to spread around. And if you do have an immune system that is already weakened because of the cold, it is practically the perfect storm to get sick in.