Pandemic influenza: using evidence on vaccines and antivirals for clinical decisions and policy making.

I nfluenza pandemics have occurred approximately every 10–50 yrs for at least several centuries, including three times in the 20th century. Influenza viruses can be highly contagious, spreading rapidly throughout regional and global populations. Furthermore, they can mutate rapidly and develop resistance to available treatments. Symptoms are generally of sudden onset, including fever, headache, cough, sore throat, aching muscles, severe weakness and respiratory symptoms of varying severity.