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(CNN) — Avian influenza, also called avian flu or bird flu, is an illness that usually affects only birds. There are many different strains of avian flu: 16 H subtypes and 9 N subtypes. Only those labeled H5, H7 and H10 have caused deaths in humans. The most commonly seen and most deadly form of the virus is called “Influenza A (H5N1),” or the “H5N1 virus.”