Animals can transmit immortal animal pathogens to people, and the animal pathogens can evolve to allow person-to-person and person-to-animal transmission. During the Spanish Flu pandemic from January 2018 to December of 1920, humans transmitted the H1N1 avian (bird) flu to swine and caused an epidemic of bird flu in swine. In 2009, swine in Mexico are believed to have started an epidemic of H1N1 avian influenza in humans, and it was then called Swine Flu.