Sudden infant death syndrome, or cot death, is by nature unexpected and, at least initally, unexplained: the term is used when an apparently healthy baby dies in its sleep and no obvious cause is found at the post-mortem examination.

Several factors are now well known to raise the risk, and this has led to a substantial decline in the incidence of cot death over the past two decades.

Babies placed on their fronts to sleep are much more likely to die suddenly than those who sleep on their backs, and loose bedding and pillows can create a small risk of suffocation. There is also a well-established link between cot death and maternal smoking during pregnancy and exposure to second-hand smoke after birth.

In Britain between 1971 and 1988, two babies in 1,000 died of the syndrome; this death rate has since fallen to one in 2,000, thanks to safety campaigns.

Much less is known about what causes cot deaths at a biological level, though common bacterial infections with Escherichia coli and Staphylococcus aureus may explain at least some of them.

The Italian research strengthens a hypothesis that a serotonin imbalance in the brainstem may be responsible. Such abnormalities appear to prevent babies from awaking to correct breathing or temperature problems.

The study has also identified a genetic route by which serotonin levels can be altered, at least in experimental mice. This supports the idea that some babies may have a genetically elevated risk.

It is not known whether genetics is the sole cause of serotonin imbalances. They may be the result of known risk factors such as exposure to cigarette smoke; or genes and the environment may both be involved.

As multiple cot deaths in one family are very rare, it has been hard to establish whether there is a genetic influence. That could soon be changed by a technique called genome-wide association, which can identify genetic variations that slightly raise the risk of medical conditions.

Regardless of whether genes are involved, parents can reduce the danger of cot death substantially. When all three known risk-factors are eliminated, the incidence falls from 1 in 2,000 to 1 in 10,000, according to the Foundation for the Study of Infant Deaths.

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