Dr. Needham, who recently evaluated the young man, said his strength and exercise capacity were below average for his age. Given his history as an athlete, they should have been above the mean. As much as Rob wants to play lacrosse again, he does not have the strength or stamina.

It remains difficult to tease out which disabilities come from the illness as opposed to the I.C.U. stay, but scientists are beginning to worry about the effects of simply being in an intensive care unit, on a mechanical ventilator that pushes oxygen under pressure in and out of the lungs, receiving doses of sedatives, narcotics and anesthetics high enough to make even healthy people stop breathing on their own. They have been particularly surprised by how quickly patients had lost strength. Now, it looks like what was lost may not completely come back, even years later.

“We are in the infancy of trying to figure this out,” Dr. Morris said.

Most patients who spend time in an I.C.U. lose significant weight.

Some are like one of Dr. Morris’s recent patients, Michelle Rhynes, 35. Ms. Rhynes, who lives in Winston-Salem, N.C., was confined to her bed for four days with bronchial pneumonia, burning with fever. At 2 a.m. on the fifth day, she collapsed when she tried to get up.

“I asked a friend to call an ambulance,” she said. “When I got to the hospital, I couldn’t breathe.”

She spent a month in the I.C.U., breathing with the aid of a mechanical ventilator, a feeding tube in her stomach. Ms. Rhynes, who stood 5-foot-6, experienced a loss in weight to 95 pounds from 140 pounds.

Or they are like Gary English, one of Dr. Needham’s patients. He lives in Baltimore, has chronic obstructive pulmonary disease and spent two months in Hopkins on a mechanical ventilator. When he got out, Mr. English, 57, who is 5-foot-9, said he weighed 78 pounds. A year later he weighs 110.