So we know that the flu goes dormant during the summer for the most part, but not why exactly (according to Wikipedia). I won’t just copy the entire article here, but it’s interesting and I highly recommend reading it. A few possibilities (C&P from the article): A recent research done by National Institute of Child Health and Human Development (NICHD) on influenza virus identified the virus as having “butter-like coating”. The coating melts when it enters the respiratory tract. In the winter, the coating becomes a hardened shell; therefore, it can survive in the cold weather similar to a spore. In the summer, the coating melts before virus reaches the respiratory tract. Research in guinea pigs has shown that the aerosol transmission of the virus is enhanced when the air is cold and dry. The dependence on aridity appears to be due to degradation of the virus particles in moist air, while the dependence on cold appears to be due to infected hosts shedding the virus for a longer period of time. The researchers did not find that the cold impaired the immune response of the guinea pigs to the virus.