One of the enduring mysteries of medicine are the reasons for the annual peak of cold and flu season during the winter months. (Note that the Southern Hemisphere has an equivalent but opposite cold/flu season corresponding to their winter from June to September.) One explanation for this seasonality is our lack of sunlight exposure during the winter, due to spending more time indoors and because winter sunlight is low-intensity due to the tilt of the earth’s axis. But what is the link between sunlight and immunity? The most popular explanation is Vitamin D, which is produced in our bodies in response to sunlight, and contributes to a properly functioning immune system. This group from Georgetown University proposes a second explanation that is independent of Vitamin D: sunlight activates the motility of T cells, which are crucial players in the immune response.