I can think of a lot of reasons why the same people would return to the hospital again and again. Diabetic episodes. Brain bleeds. Kidney stones. All of those have caused repeated hospitalizations for friends.
Chronic inflammatory colitis... alcoholism causes bleeding. Addiction to prescription drugs. Falls. Infectious diseases. People over the age of 55 have a high incidence of pneumonia.
http://www.aafp.org/afp/2001/0115/p257.html