Medicine: Teddy's Tiny World

Teddy DeVita, 13, seems like a normal teenager. Darkly handsome with brown hair and twinkling eyes, he sings folk songs, plays the guitar, performs card tricks and is fascinated by the Old West. But Teddy is no ordinary youngster. His world consists largely of a small (8% ft. by 10 ft.), near-sterile chamber at the National Cancer Institute (NCI) in Bethesda, Md., where he has lived in isolation for the past 31/2 years.

The youngster is the victim of severe aplastic anemia, a rare, puzzling disease in which the bone marrow loses its ability to...