It's a disease of the vessels in the lungs. They get occluded or blocked and O2 just doesn't flow the way it should. No one knows what causes it. There is no cure for it. My father in law has had severe shortness of breath for several years. Every lung test has been done. Finally about a year ago he was diagnosed. He is on 8 to 10 liters of O2 at all times. He does fair until he exerts any energy or walks. He doesn't get enough O2 to the brain and will pass out. He lives with us because he is unable to do anything much for himself. And because when he gets up to walk, sometimes he gets weak or when he passes out he falls. He was in pretty good health up until about three yrs ago. It happened pretty much suddenly. It has progressed really fast. He is terminal, and they say it will just keep getting worse until he dies. It's very hard to watch someone you love go from a healthy, happy, outgoing person who was working every day and into many organizations and clubs, just sudenly be unable to do anything. He is at a plateaau right now and is able to go play cards with friends once a week. Some of us has to go and stay with him to change his O2 tanks and keep an eye on him. He is in a wheelchair, but can't do it himself. Someone has to push him. It's a terrible disease!Just eating or combing his hair is very tiring for him and he struggles to breathe.