It happened to the son of friends of mine, when he was 23 or so (This was four years ago now, I believe). He'd just moved into a new housing situation with roommates, in an old house, and somehow during the middle of the night while he was sleeping, a fire broke out on the porch. He was overcome with smoke, and by the time the fire department rescued him, he'd suffered severe and irreversible brain damage. After months in the hospital ICU in a coma, and then for nearly a year in a brain rehabilitation hospital, where he was cared for and received therapy, he was moved to a nursing home, where he stayed, unable to walk or speak, for more than a year. Recently (for complex insurance reasons and regulations), his unbelievably dedicated parents (they have done everything in the world for him to receive testing, treatments, therapies, anything possible) secured him a place in a group home, so that he could qualify to receive more physical therapy.
This was a brilliant and vital young person, who will never be able to care for himself. But he's alive, and he is loved by many of us. We still pray for a miracle.
You can only imagine what kind of medical needs and costs such a thing incurs. Tragedies happen. I hope they never happen to you or yours, but they do happen. Be prepared. (Obviously, I believe much of the long-term care needed has shifted to Medicaid and SSDI.)