As a medical student, I was taught that colds and flu are more common in winter because people crowd together indoors during cold weather. That's certainly part of the explanation - the average child 'sheds' virus for 11.4 days and the average adult 10.1 days after a cold. Since viruses are picked up either from breathing in when someone sneezes, or by touching a surface someone has sneezed on to and then touching your eyes, mouth or nose with your hands, close contact makes for more infections.
This is a companion discussion topic for the original article at https://patient.info/news-and-features/can-you-catch-a-cold-in-summer