Researchers from Manchester Royal Infirmary say babies who died from sudden infant death syndrome (SIDS) were found to have a higher rate of the Helicobacter pylori infection than other children.
Although scientists are still unsure about how the bacterium could cause cot death, Dr Jonathan Kerr, who led the team, urged parents not to lick their children's pacifiers or dummies before replacing them in their mouths and to make sure they keep their hands clean.
"Parents need to know that good hygiene is important and that the organism can be passed from their mouth to their baby's," Dr Kerr said.
In the past decade the number of cot deaths in Britain has fallen by 70 percent, from 1 008 to 284 per year.
But it is still the main cause of death for babies more than a month old, killing almost one a day.
It is largely unknown why seemingly healthy babies suddenly fall victim to the condition, although Department of Health guidelines such as putting babies to sleep on their back and not smoking near young babies have reduced the likelihood of a baby dying in the first year of life.
The researchers, who published the study in the Archives of Disease in Childhood, examined the bodies of 32 babies under seven months old who had died from SIDS and compared them with eight babies who had died of other known causes.
Helicobacter pylori is a bacterium that causes a stomach inflammation called type B gastritis.
It is associated with peptic ulcers and gastric cancer, and affects about two percent of the population in the developed world.
Causes inflammation
Two genes indicate the likelihood of Helicobacter infection. One or both genes were found in the tissue samples of 28 out of the 32 babies who died from SIDS, indicating a Helicobacter infection rate of 88 percent, far higher than the two percent rate that is normal in the developed world.
One theory about how the bacteria could cause cot death is that the infection causes inflammation and large amounts of urease, the chief waste product discharged from the body in urine.
Large amounts of urease could trigger production of ammonia in the baby's underdeveloped immune system.
Unable to be dispelled by the liver, the ammonia stays in the baby's system and could be fatal, Dr Kerr said.
Joyce Epstein, director of the Foundation for the Study of Infant Deaths, said: "These are interesting findings that may help explain why overwrapping, front sleeping and minor infections may create difficulties for babies."