I spent four months in hospital earlier this year and I can vouch for the fact that the roast isn’t all that bad, although after four months the menu does become somewhat monotonous.
There are several things to consider though both good and bad.
Firstly the food given to the patients is usually different to that in the canteen, so if you’re eating there you’re not likely taking food from them, plus patients in the hospital I was in usually order their meals in the morning so those would already have been accounted for anyway.
Secondly, parking charges would probably have been higher than the cost of the roast, so you won’t really have saved any money. Added to which you could be taking up space in the hospital car park for someone who might wanted to have visited a relative and they in turn would have to spend more time looking for parking. If you started a trend this could potentially cause issues.
Re illnesses, I would be less worried about what you might catch, than I would be about what people might catch from you..
people in hospital are already sick and so their resistance is lower. And if you’re the type who thinks of casually dropping into hospital for dinner then you’re likely the type who is fairly casual about your health.
The hospital I was in is highly specialised and there are numerous immune suppressed patients, It’s inevitable that those patients will at some point come into contact with infection, but going down to the canteen is a part of their re-introduction to the outside world, if numerous outsiders started going there it could start to be considered to not be the safest place for newly immune suppressed patients to start going, and they’re the ones who actually are there for a reason.
I can’t vouch for other hospitals, but I imagine they would have some similar issues.