For example, during the 2009 swine flu outbreak in the United States, the CDC advised physicians to "consider swine influenza infection in the differential diagnosis of patients with acute febrile respiratory illness who have either been in contact with persons with confirmed swine flu, or who were in one of the five U.S. states that have reported swine flu cases or in Mexico during the seven days preceding their illness onset."
Sebagai contoh, selama wabah flu babi 2009 di AS, CDC menganjurkan para dokter untuk melihat "apakah jangkitan flu babi pada pasien yang di diagnosa memiliki penyakit pernapasan akut memiliki hubungan dengan orang yang di tetapkan menderita flu babi, atau berada di lima negara bagian AS yang melaporkan kasus flu babi atau berada di Meksiko dalam jangka waktu tujuh hari sebelum bermulanya penyakit mereka."