Using the guinea pig model, we have previously shown that the aerosol transmission of a seasonal human influenza virus is blocked by humid (80% relative humidity) or warm (30°C) ambient conditions. In contrast, we found that transmission by a contact route proceeded at high efficiency despite increased temperature or humidity. Based on these findings, and the observed seasonal behavior of influenza viruses in various regions of the world, we hypothesize herein that the predominant mode of influenza virus transmission differs in temperate and tropical climates. Specifically, we predict that aerosol transmission predominates during the winter season in temperate regions, while contact is the major mode of spread in the tropics. With this idea in mind, possible explanations for the current summer-time spread of swine-origin influenza viruses are discussed.