True about epidemology of influenza -

Influenza is an acute respiratory tract infection characterised by sudden onset of chills,malaise,fever,muscular pain anx cough. Influenza A strains are pandemic.Incubation period 18 to 72 hours Major reservoirs exists in animals and birds Reference; Park&;s Textbook of preventive and social medicine, 24th edition.Pg no. 163,164