It is still an unsolved mystery as to why flu is seasonal. Where does it go in those summer months? Scientists studying modeling techniques that were originally used to model fish population dynamics based on climate are now testing the model on predicting certain aspects of flu. The scientists believe that using data on temperature and humidity could help predict flu outbreaks.
They concluded in study published in September that humidity plays a big role in predicting flu outbreaks. This conclusion may help explain why there is no flu season in tropical areas that experience low fluctuations in humidity.
Global Environmental Drivers of Influenza. http://www.pnas.org/content/113/46/13081