Even a cold could kill

About 40 babies are born in Britain each year with severe combined immunodeficiency, or SCID.

It is a genetic condition in which a baby's defence or immune system is not properly formed and it means that an infection - even a common cold - can be fatal.

Babies with the condition, which stops the white blood cells which fight infection from being produced, must be isolated from the outside world - hence the name "baby in the bubble".

They are likely to die within a year without a bone marrow transplant.

There is one chance in 50,000 of a baby being born with the condition.

The immune system consists of cells called lymphocytes, made in the body's marrow. In a baby with SCID, the bone marrow is unable to make enough of the right type of healthy cells. A bone marrow transplant can often cure the condition completely as long as it is diagnosed quickly enough.

But it is a balancing act. The marrow may start attacking its new host, causing "graft versus host" disease.

If the baby has an infection, the new marrow will immediately attack the infection. This may endanger the baby if the infection has a hold on a major organ such as the lungs.

The first case of SCID in the UK was reported in 1950 and, before the advent of modern medicine, babies with the disorder would rarely live beyond their first year.