It’s not about the weather itself, it’s about the fact that in winter people stay indoors more, in confined unventilated spaces.
It's actually both. With some viruses like the flu the membrane of the virus toughens in cold weather and they last longer out of the body... But yes, people being indoors more helps spread it too.
Where I am it's so hot in the summer people tend to stay indoors and cases of flu peak a little in summer too.
