Following adaptation and host switch, influenza A viruses have the potential to cause epidemics and pandemics in humans. Minor changes in HA and NA structure (antigenic drift) occur frequently, which enables the virus to cause repetitive outbreaks (i.e. seasonal influenza) by evading immune recognition. Major changes in HA and NA structure (antigenic shift), which are caused by genetic reassortment between different influenza A subtypes (e.g. between human and animal subtypes), can instead cause large regional/global pandemics. Due to the emergence of antigenically different influenza A strains in humans, four pandemics occurred in the 20th century alone.