Theories

Why do most people come down with the flu only during cold weather?

While researchers have no hard evidence to answer it, tantalizing theories and observations abound about cold weather and the flu.

Theory 1 - People socialize indoors in winter. Children and adolescents return to school, filling classrooms. Droplets of flu virus spread easily to people in close quarters.¹⁰⁸

Theory 2 - Ultraviolet exposure has been effective at killing viruses in the laboratory. Perhaps the UV rays of summer keep the flu virus at bay then.¹⁰⁹

Theory 3 - Flu surveillance almost ceases during the summer. So outbreaks of flu most likely would go unregistered.110 However, researchers have reported isolated summertime outbreaks in groups of people in relatively enclosed places.¹¹¹

Most recently, a summertime outbreak of flu occurred on a cruise ship in Alaska. Travel places tourists from different hemispheres in contact; an infected person from Australia, where it is winter, could infect a person from the U.S. where it is summer.¹¹²