Influenza is a contagious respiratory illness. It is characterized by a sudden onset of fever, headache, myalgia, malaise, prostration, non-productive cough, sore throat, and rhinitis. It has usually a short incubation period, from one to three days.
Most people recover within one to two weeks without any medical treatment. However, severe illness and death during annual influenza epidemics occur primarily among the very young, the elderly and those with other serious medical conditions.
There are three types of seasonal influenza – A, B and C. Among many subtypes of influenza A viruses, currently influenza A(H1N1) and A(H3N2) subtypes are circulating among humans. Type C influenza cases occur much less frequently than A and B. That is why only influenza A and B viruses are included in seasonal influenza vaccines.
Seasonal Influenza is in the list of category C of notifiable diseases to be reported according to the Law of Communicable Diseases Prevention and Control of China.