It would seem like the cold weather would kill off the viruses and germs that cause the cold and flu but instead flu and cold viruses and other germs actually thrive and survive much easier and long when it's colder out.
The lower the temperature the higher survival rate of the germs and the hotter the temperatures the less time the cold and flu viruses and germs will survive.
That's why when you cook food it kills bacteria which makes the food safe to eat because the heat kills the germs and bacteria.
During hotter weather the cold and flu germs and viruses don't survive as easily so that's one reason why people get less colds and flues during the spring and summer months but you still can catch the flu and colds during the spring and summer months but you're less likely to catch the cold and flu during warmer weather.