Exercise strengthens the immune system by increasing the production of white blood cells that helps to fight off colds and flu. Nutritionist Susie Burrell says the cooler months are no excuse not to exercise. "We need to embrace the cold weather and even rug up and exercise outdoors like they do in the US and the UK," she says. If you really do struggle pulling on your gear when it's dark and cold outside, go for a walk jog during your lunch break.