why should smoking and spitting be forbidden in hospitals?

This, and your 'Christmas' question, tend to indicate that you may be Chinese.
If so, I suggest you research the concept of 'culture shock'.
(I presume you know that smoking and spitting are both health hazards which have become socially unacceptable in 'the west').

Live oxygen and the need for a sterile environment.

There was a young man from Darjeeling
Who boarded a bus bound for Ealing
The sign on the door said "Don't spit on the floor"
So he stood up and spat on the ceiling.

And after he spat he felt really quite dim,
Cause the joke in the end dripped on him

A man in a hospital ward
Lit a ciggie because he was bored
As he started to cough
His gas bottle went off
And into the car park he soared.