If you happen to inhale or swallow a fly nothing bad will happen to you.
Most times you'll cough and spit out the fly that you got into your mouth before you swallow it so it never goes down into your stomach.
But if you did happen to swallow the fly and it made it's way down into your stomach then your stomach acid will just digest and kill the fly and then you'll poop out whatever is left of the fly.
Nothing bad should happen to you if you do swallow a fly and the fly should not make you sick if you did swallow a fly.
I've swallowed several flies throughout the years and nothing has happened to me.