Staying in one position for a prolonged time. Pohibits the blood flow to that particular area and the skin starts to breakdown.

What witchway says is true, but you must also essentially consider the patients underlying medical issues, especially  diabetes and heart disease/impaired blood flow throughout the body. There are others I'm sure, but I'm not an expert. Consistent and frequent repositioning of the body, up and ambulatory/walking if possible  (for better blood circulation)  and proper, attentive care  given  by care-givers as well as doctors  given at the first signs of a bed sore can manage it well or even stave it off.  Best of luck.

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