Doctors have come a step closer to understanding what causes cot deaths. In a 10-year study of sudden unexplained infant deaths dealt with at Great Ormond Street hospital in London, the researchers found a link with bacterial infection.

At present, they caution, the link is merely an association. They cannot be sure that infections found during autopsy were what caused the deaths. However, the study does go some way to confirming suspicions among medical professionals that bacteria such as Staphylococcus aureus and Escherichia coli are sometimes the cause.

"It's an idea that year by year has a bit more evidence ... this is another step in that process, but I think quite an important step," said Dr James Morris, an expert in sudden infant death at Lancaster Royal infirmary, who was not directly involved in the study.

"We've now really got a reasonable hypothesis that these organisms are causing sudden infant death, at least in some cases."

In 2005, there were 268 unexplained infant deaths in England and Wales, a rate of 0.41 deaths for every 1,000 live births. To avoid cot death, parents should lay their child on its back to sleep, not smoke in the house and not let it sleep in the bed with them, said Morris.

To test the bacterial link, the Great Ormond Street team reviewed 546 autopsies from cases of infants under a year old who died between 1996 and 2005. They found that those with an unexplained cause of death were more likely to have a bacterial infection than those who had died of something other than an infection. The study is reported today in the Lancet.