Researchers have pinpointed two common bacteria that may contribute to cot deaths, even when infants show no sign of tissue damage.
Post-mortem tests on more than 500 babies found high levels of Staphylococcus aureus and Escherichia coli in babies who died for unexplained reasons, a team from Great Ormond Street Hospital for Children in London said on Friday.
One explanation could be that the bacteria release deadly toxins, which damage the young heart, lungs or nervous system.
But bacterial growth may also be a secondary effect of other known risk factors like over-heating, parental smoking and lying a child on its stomach.
Sudden unexpected death in infancy (SUDI) is a leading cause of death in babies under a year old, yet its root cause remains a mystery. Healthy looking infants can often die in less than an hour.
The latest findings in the Lancet medical journal suggest underlying infection could be an important component.
"You've got to be very careful how you interpret this data," Nigel Klein, one of the researchers, said in an interview.
"But we did find an increased number of bacteria grown from particularly the lungs and spleen in infants who died unexpectedly without a known cause."
Cases of S. aureus and E. coli were significantly more frequent in the group of babies whose death could not be explained than in those who died of non-infective explained causes, such as congenital abnormalities.
Both S. aureus and E. coli are classed as "group 2 pathogens", which are known to cause septicaemia without obvious damage to tissues in the body.
Alan Craft, professor of child health at the University of Newcastle, said the findings were important but there might not be a simple answer.
"The bacteria found are ones which are in all of our bodies most of the time and there is nothing that can be done to avoid them," Craft said.
