In Japanese, the new flu that’s been spreading around the world is called buta influenza (“pig influenza”). In English, it is most commonly called swine flu. Swine is a general word for pigs. Flu is the commonly-used short form of influenza.
In English, we have many different expressions using the word “pig.” One common expression is “when pigs fly!” It’s impossible for pigs to fly. Therefore, when someone asks you when you’re going to do something, if the answer is never, you can say, “When pigs fly!”
If Mayumi asks me, “When are you going to start eating natto?”
I can answer, “When pigs fly!”
After that, if she ever caught me eating natto, I guess she could say, “The pig flew!”