"Just like fire drills in school, casts rehearse for this. They are well prepared."
As Tag said, this is not at all true. At least, not on Broadway. Some theaters are better prepared than others (specifically stage and house managers), and every situation is dealt with on a case to case basis. Sometimes a patron can get out under their own power, otherwise the show must be stopped til the first responders get there - and even then, sometimes they will need to care for the patron in situ before moving them.
I have never heard of a cast rehearsing for a medical emergency. Not ever. Sometimes it is an actor who stops the show (Tucci famously did it nude, in Frankie and Johnny), but far more often, it's the stage manager, who has been notified by an usher or other front of house worker. Often another patron. The SM will announce over a god mike that the show will be stopped, and the house lights brought up.
The extent of the cast being "well prepared" consists mostly of walking off the stage when someone tells them to. What is there for them to rehearse for?
This also happens far more often than you would think.