A Joke!

I'm working from home today ailing from one of Hong Kong's random stomach bugs which require proximity to home comforts (probably too much information?) anyway... My mom left a message asking me to ring home, and when I did Dad told me a joke... very unlike dad (the joke that is) but I'm still laughing at it, not really because it's that funny, but because my staid and responsible father told it (and said he really didn't just say that afterwards!):

Why is PMS called PMS?
Because Mad Cow Disease was already taken