The phrase "when pigs fly" (alternatively, "pigs might fly") is an adynaton—a figure of speech so hyperbolic that it describes an impossibility

https://en.wikipedia.org/wiki/When_pigs_fly

The H1N1 flu, sometimes called swine flu, is a type of influenza A virus

https://www.mayoclinic.org/diseases-conditions/swine-flu/symptoms-causes/syc-20378103