Swine flu is caused by swine-origin H1N1 virus. Of the 15 HA subtypes found in birds, only a few have been transferred to mammals (H1, H2, H3, and H5 in humans; H1 and H3 in swine; and H3 and H7 in horses). The same pattern holds for NA; nine NA subtypes are known for birds, only two of which are found in humans (N1, N2). This pandemic strain of H1N1 causes a broad range of clinical symptoms ranging from typical symptoms (fever, malaise, myalgias, cough, sore throat, rhinorrhea, shoness of breath) commonly accompanied by gastrointestinal manifestations (especially diarrhea) to fulminant viral pneumonia. Ref: Brooks G.F., Carroll K.C., Butel J.S., Morse S.A., Mietzneron T.A. (2010). Chapter 39. Ohomyxoviruses (Influenza Viruses). In G.F. Brooks, K.C. Carroll, J.S. Butel, S.A. Morse, T.A. Mietzneron (Eds), Jawetz, Melnick, & Adelberg's Medical Microbiology, 25e.