Tell them that HIPPA said you didn't need to. lol A supervisor I told I was ill and wouldn't be in for work was rude once.
She demanded to know why I was sick...I told her according to HIPPA...that's all she needs to know that I am too ill to come to work.

I tell my SIL that a lot if she asks how I'm feeling...HIPPA says I don't have to tell ya...of course she know its the 'joke'.

Still, sometimes it would be nice just to tell them exactly why you don't feel like doing something.