If it was something your child could catch that was some deadly disease you would know about it already. Since i'm sure something like a rampid plague of ebola hasn't happened in your daycare, do you really feel it is your business?
People have the right to their privacy, especially the person in the ambulance and the family of the person in the ambulance. I honestly don't think it was any of your business and there will probably be a notice sent home to let the parents know what they need to know (what they NEED to know, not what they WANT to know).