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

Why does flu occur year round in the tropics, and more often during rainy seasons?

These are just two questions that intrigue virologists.¹⁰⁵

The traditional "season of suffering" varies by location. In the Northern Hemisphere, flu can strike people from October through April, but it usually peaks between December and March, and actually hits a given community only for an average period of eight weeks. In the Southern Hemisphere, the influenza months are reversed with a season that runs from April to September.¹⁰⁶

In both Hemispheres, a virulent strain of flu can sweep an entire country quickly. Milder Type B viruses or mixed outbreaks of A and B viruses tend to travel around a country more slowly.¹⁰⁷