One sneeze and influenza is drifting through the air, plastered across palms of hands and around door handles, poised for its next victim. How long can the virus survive outside a living host? The answer to this question depends on ambient environmental conditions. Shaman and Kohn showed experimentally that low absolute humidity (grams of water per cubic meter of air), which tends to prevail during temperate winters, improves the airborne survival of influenza viruses within aerosolized drops and favors transmission. Shaman et al. modeled how changes in absolute humidity have driven the seasonal peaks and troughs of influenza in the United States during a 30-year period. Epidemics were correlated with the onset of anomalously low absolute humidity, and variations in absolute humidity affected the occurrence of outbreaks during any one season. Thus, it may be just as feasible to forecast short-term influenza risk as it is the weather.
Proc. Natl. Acad. Sci. U.S.A. 106, 3243 (2009); PLoS Biol. 8, e1000316 (2010).