# Case Definition for Influenza A (H1N1) Virus Infection (Swine Flu)

After identification of novel influenza A (H1N1) virus infection in Mexico, a case definition was developed. The initial definition of suspected novel influenza A (H1N1) virus infection included any hospitalized patient with severe acute respiratory illness. On May 1, 2009, this definition was expanded to include any person with acute respiratory illness defined as fever and either sore throat or cough. On May 11, 2009, the definition of suspected case was changed again to include any person with: