Severe Combined Immunodeficiency, typically abbreviated as SCID, is a genetic disorder which cripples the immune system. It is also known as the "boy in the bubble" disease because its victims must live in a completely sterile environment.

The disorder is caused by an inactive enzyme (adenosine deaminase) necessary for the synthesis of DNA. This results in the inability to produce white blood cells, which are part of the body's immune defences.

Although David, the original "boy in the bubble" died after he decided enter the outside world, SCID is now being treated with gene technology. Two American girls, well known for their disease, are living normal lives in everyday environments.

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