I don't suppose a patient could later sue a hospital for giving him or her hospital-acquired pneumonia. It's not like the hospital can do any more about staph bacteria than a school or restaurant or other public building. When I was in the hospital for minor surgery, my nurse told me that some people are prone to ventilator associated pneumonia, and the nurses can't just stop the ventilator treatments because of it.
There was also a condition called aspiration pneumonia, often caused by patients inhaling their own vomit into their lungs. It was considered a chemical kind of pneumonia, not the same as the kind triggered by pneumonia bacteria. Let's just face it, if you have to spend time in a hospital, you're going to have to accept the fact that all of those risks are going to be present. No hospital can completely eliminate every single toxic substance in the building.