Train Passengers with Flu Masks (IMAGE) University of Arizona Caption Flu pandemics occur when virus strains circulating predominantly in animals acquire mutations that allow them to jump into and spread among humans more readily. Credit Eneas De Troya/Wikimedia Commons) Usage Restrictions This visual may only be used in conjunction of reporting/posting of this news release. Credit must be stated as indicated. License Licensed content Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.