Flu outbreaks either appear seasonally or as pandemics. This has nothing to do with the level of danger a virus poses.
Seasonal flu has been known for centuries. Outbreaks are caused by person-to-person transmission, such as coughing, sneezing or touching.
Flu pandemics affect the entire world. New viruses pose more of a threat because human immune systems take time to build resistance.
Animals are also susceptible to flu viruses.