[Riflema]

Well, I hate how Yanks refer to it with the definite article. Like there's only one hospital, "THE" hospital.

In this context, it means a condition, not a geographic location, as in "in bed". We would not say (WJC excluded) "in a bed" would we?

[bagster]

"In hospital" is equivalent to saying, "I got in car."

There are plenty of cars but it's understood that you can only get in one car at a time. Same with hospitals.

[bagster]

As far as being a condition. One is “in the car” or “in a car” not “in car.”