Influenza seasonality: lifting the fog.

S easonal variation in the incidence of communicable diseases is among the oldest observations in population biology, dating back at least to ancient Greece, yet our understanding of the mechanisms underlying this phenomenon remains hazy at best. Influenza is perhaps the seasonal disease of most profound interest, because it is responsible for much of the seasonal variation in other infectious and noninfectious causes of morbidity and mortality (1–4). Influenza virus activity displays strong seasonal cycles in temperate areas of the world and less defined seasonality in tropical regions (5), suggesting that environmental factors may drive seasonal patterns. In a recent issue of PNAS, Shaman and Kohn (6) move us a step closer to understanding influenza seasonality by clarifying the impact of environmental factors on influenza virus transmission and survival. The authors demonstrate an impressive statistical association between vapor pressure, influenza transmission, and virus survival.