Why do I always get sick on holiday?

“Whether influenza, the common cold or other viruses, it’s really quite clear that mass transportation contributes to transmission,” William Schaffner, a professor of preventive medicine at Vanderbilt University Medical Center, told Quartz. “It’s not the only way, but it’s a major way.” The makeup of the influenza virus allows it to multiply and spread best when air is dry and cold. So it's not the cold are that's causing you to fall ill, it's the virus.