The influenza virus (a germ) causes influenza or "flu." The flu is an infection of the breathing tubes and the lungs. In recent years, flu viruses that at first only infected birds have begun to infect humans. One of these strains is called avian influenza (A/H5N1 subtype) or "bird flu". Although no human cases of bird flu have been diagnosed in the United States, this strain has caused severe illness and death in several hundred people since late 2003. .