In Night, why does Elie have to go to the hospital?

By Chapter 5 of Night, Elie's foot, which begins simply swollen, is infected.  After consulting with the doctor, Elie is told that he needs an operation; without it, his foot will likely need to be amputated.  Elie goes to the concentration camp hospital.  On the bright side, there are better rations of food there.  On the other, there are also more frequent selections, and Elie fears that he will be selected without the opportunity to reunite with his father.

When Elie hears a rumor that the Russian army is close by and that the Germans intend to kill everyone in the hospital before surrendering, Elie sneaks out of the hospital and proceeds to meet up with his dad.  He feels that he has made the right move, although he finds out later that those who remained in the hospital were simply liberated several days later.