The viruses are further subdivided into distinct subgroups with one or another predominating during an annual epidemic. When a new strain enters the population, the result can be a pandemic with excess morbidity and mortality as occurred in 1918.
Influenza can cause fever, chills, cough, sore throat, headache, and muscle aches.
The influenza virus is usually passed when an infected person coughs or sneezes and another person inhales droplets containing the virus. Although most people are ill for only a few days, some have much more serious illness and need to be hospitalized. Thousands of people die each year from influenza-related complications. Most influenza-related deaths are in the elderly. In addition, only a split-virus influenza vaccine should be given to children 6 months to 12 years of age.