A very thorough answer to something I’ve been wondering about
There are a few reasons.
First, cold weather directly affects blood flow. During the flu season of 2005, an experiment was performed to test the idea that being cold can make you sick. 90 people kept their feet in a bowl of ice water for 20 minutes, while a control group of 90 people put their feet in an empty bowl for 20 minutes. Over the next 5 days, 29% of the group with chilled feet developed cold symptoms, compared to only 9% of the control group.