In an analysis of the spread of influenza in the US over 30 years, Viboud et al. observed a consistent early onset of the epidemic in California, which is the most populous state in the U.S. [17] . Interestingly, the onset in California was earlier than in 3 populous Eastern states, suggesting that in addition to population factors also geographical and climate factors, such as:, mountain ranges, plains, lakes, and predominant wind direction may drive early epidemic activity. Some of these factors may be involved in a west-east spread in Europe: the western of Europe is the most populated part and factors that contribute to spread, such as commuting and airline travel consequently are more intense in the western than in the eastern part of Europe [18] .