Что значит эти предложения: an acute infectious disease…?

Что значит эти предложения: an acute infectious disease of the respiratory tract caused by the influenza virus. Included in the group of acuterespiratory viral infections (ARI). Periodically distributed in the form of epidemics and pandemics. Currently identified more than 2000 variants of influenza virus of differing antigenic spectrum WHO estimates of all variants of the virus during seasonal epidemics in the world die each year from 250 to 500 thousand people (most of them are older than 65 years) , in some years the number of deaths could reach a million