Seasonal flu patterns in temperate and tropical regions may both be driven by specific humidity

Every year, about a billion people worldwide contract influenza, leading to as many as 650,000 deaths. Outbreaks are often seasonal, and their pattern varies by climatic zone. In temperate regions, flu season is in the winter, while in the tropics, constant low-intensity activity is typically punctuated by two annual peaks—or a surge during the rainy season.