The flu virus doesn't evolve into a new species of flu virus .... It adapts to our medicine. You know scientists who have labeled everything as evolved are the ones that say the flu evolves. Newly emerging or “re-emerging” viral diseases continue to pose significant global public health threats. Prototypic are influenza viruses that are major causes of human respiratory infections and mortality. Influenza viruses can cause zoonotic infections and adapt to humans, leading to sustained transmission and emergence of novel viruses. Mechanisms by which viruses adapt in one host, cause zoonotic infection, and adapt to a new host species remain unelucidated. Here, we review the adaptation of influenza A viruses in their reservoir hosts and discuss genetic changes associated with introduction of novel viruses into humans, leading to pandemics and the establishment of seasonal viruses.