Why Colds and Flu Peak in Winter

Ever notice how colds and flu seem to arrive with the chill of winter? It’s not just coincidence — viruses thrive when temperatures drop. From October through May, respiratory infections surge in the U.S., with flu season often peaking in February, according to CDC data. The Southern Hemisphere experiences its spike during winter (April–September), while tropical regions see cases year-round.

So, why exactly do viruses spread more easily in the cold months?