Babies born without immune systems quickly become ill during weaning when the protection of their mother's milk – which contains maternal antibodies – begins to wear off. Without transplant medicine of some kind, the only way of keeping these children alive is to cocoon them in a sterile environment free of potentially lethal microbes.
In the Sixties and Seventies, hospitals put babies with severe combined immune deficiency (SCID) in plastic "bubbles" where the the air is filtered and direct contact with the outside world is minimised.
One of the most famous of these so-called "bubble babies" was a Houston boy, David Vetter, who spent 12 years of his life inside a plastic sphere aerated with filtered air. He became known as "David the bubble boy", and his predicament was made famous in a song by Paul Simon. David Vetter died in 1984 when, at his insistence, doctors tried a bone-marrow transplant.
Since then, transplant operations have improved, provided that doctors can find suitably matched donors of either bone marrow or stem cells from umbilical cord blood. The drugs used to fight the opportunistic infections that normal babies usually shrug off are also better than they were.
At Great Ormond Street Hospital in London, doctors house SCID babies in cubicles where the air is filtered and put under a slight positive pressure, to prevent outside air seeping in. One or two doctors or nurses, with either the mother or the father, can join the baby in his or her sterile world.
Improved antibiotics sometimes allow these children to go home for limited periods, but they are not allowed to mingle freely with their siblings or other children.
Even though they are not "bubble babies" in the traditional sense of the phrase, they are still isolated from the rest of the world and as they grow up they are denied the social interaction that other children take for granted.
Now that gene therapy has been proved to work with SCID babies, there is hope that the prospect of growing up in a "bubble" will be consigned to medical history.