Avian flu (also "bird flu", "avian influenza", "bird influenza"), means "flu from viruses adapted to birds", but is sometimes mistakenly used to refer to both other flu subsets (such as H5N1 flu) or the viruses that cause them (such as H5N1). "Bird flu" is a phrase similar to "Pig flu", "Dog flu", "Horse flu", or "Human flu" in that it refers to an illness caused by any of many different strains of flu viruses such that the strain in question has adapted to the host. "Avian flu" differs in being named after an entire vertebrate class with 8,800–10,200 species.