Geoinformatics plays an important role in the study and control of infectious disease outbreaks, and it includes techniques such as geographic mapping and location-based alert services (16, (24) (25) (26) . As was recognized previously, the international spread of pandemic influenza and severe acute respiratory syndrome was largely related to air travel (27) (28) (29) . Our study confirms that air travel and transportation routes accelerated the spread of pandemic influenza between counties in mainland China. Air travel and travel by national highways and freeways especially appeared to play a role, whereas railways were less important. In mainland China, trains are mainly used for occasional long-distance travel, whereas highways are more often used for daily or weekly commuting, especially because bus schedules are more flexible. Our previous study on the geographic spread of the severe acute respiratory syndrome epidemic in China also demonstrated that domestic travel along national highways played a more important role than travel by railway (30) . Data from the National Bureau of Statistics of China show that passenger traffic by highways in 2009 was 18.2 times that of railway travel (9) . Transportation by highway remains an important mode of travel between Chinese cities and provinces and therefore is a potential target for controlling any future emerging airborne infections.