At irregular intervals, usually of a few decades, a new influenza virus emerges which is novel to all or most people which means that there can be little specific immunity among humans except for older people who may have met unless they met the same virus.
V nerednih intervalih, navadno vsakih nekaj desetletij, se pojavi nov virus gripe, ki je za vse ali večino ljudi nov, kar pomeni, da obstaja med ljudmi le majhna specifična imunost, razen pri starejših, ki so se z enakim virusom morda že srečali.