The nutrient in question is vitamin D. Your skin can make it from the sun in summertime, but that’s not an option for some people. And it’s certainly not possible when you’re in the hospital.
The Korean researchers point out that “vitamin D deficiency in neurosurgical intensive care unit patients is an important issue… we found that vitamin D deficiency is linked to immunological status.” So, unless you get enough, your immunological status can suffer, too. That leads to more illness.
http://easyhealthoptions.com/alternative-medicine/the-vitamin-they-give-to-intensive-care-patients/