Of the hundreds of strains of avian influenza A <e1>viruses</e1>, only four have caused human <e2>infections</e2>: H5N1, H7N3, H7N7, and H9N2.
Of the hundreds of
Of the hundreds of strains of avian influenza A viruses, only four have caused human infections: H5N1, H7N3, H7N7, and H9N2.
<triplet> viruses <subject> infections <object> Cause-Effect(e1,e2)