When we get sick in the winter time it's not because of winter itself but instead it's because the cold and flu virus is more active during the winter.
Not only that but during the winter months we are cooped up more indoors around people who have the flu virus or cold virus so it's easier to catch a cold or flu during cold weather than it is when it's warmer out.
When it's nice and warm outside some people have their windows open and are outside more so the viruses are as easily able to get get into peoples systems to make them sick.
You can catch the cold or flu virus at anytime of the year including spring and summer months but you're more prone to get sick in the winter.
You cannot get sick with the cold or flu from just being out in the cold weather without a jacket or just being out there because you need to have the flu or cold virus to actually infect you with.