This means that their stays in hospital have had harmful effects upon them. The author calls the totality of the illnesses caused by a hospital admission the post-hospital syndrome, which is to say “an acquired, transient condition of generalized risk.”
Irrespective of their original condition, patients who have been in hospital often return with heart failure, pneumonia, infections of various kinds, gastrointestinal disturbances, mental illnesses such as confusion and paranoia, metabolic upsets, and trauma. The hazards of hospital are evidently various and often severe.
But what are the hazardous factors? Patients are often deprived of sleep, because of their medical condition, the unfamiliarity of their surroundings, and the constant interruption of sleep by noise. Deprivation of sleep, or reduction in its quality, has various harmful effects: on the heart, on the immune system, on blood coagulation, and on physical co-ordination. A harmful effect on the latter increases the risk of trauma caused by falls, for example; reduction in immune functioning predisposes to infections (and in hospital there are a lot of possible infections around for a weakened person to catch).