In a typical case an apparently healthy baby is put to bed in a cot or pram and when next looked at is found to be dead.
Numerous theories have been put forward to explain why this happens, but no-one has come up with a definitive answer. It may be that there is more than one cause.
Allergies, breathing problems during sleep, and passive smoking have all been associated with cot death.
There has also been much controversy over whether certain substances used as fire retardants in mattresses can harm the baby.
The Foundation for the Study of Infant Deaths (FSID) said more babies died from the distressing condition than meningitis, road traffic accidents, leukaemia, or other forms of cancer.
Seven babies died for no apparent reason every week in the UK, and cot death was the main reason for death in infants over one month old, it said.
Cot death is most common among babies aged between four weeks and one year. Boys, twins and babies with low birth weight are more likely to suffer.
Sudden infant death happens more during the autumn and winter months, and more often to bottle-fed babies.
One of the most puzzling aspects of cot death is that there appear to be no warning signs of any kind. Research has shown there could be subtle warning signals in the preceding few days which might be missed.
These may include a cold, snuffles, listlessness, drowsiness or a breathing difficulty.
While a cause is found for about a third of sudden infant deaths, in two thirds of cases a routine autopsy is completely unable to determine why the baby has died.
One theory suggests that excessively high body temperature might be a factor. Some babies may die as a result of a previously unrecognised congenital heart defect, or a heart abnormality may emerge as the baby grows.
Putting babies to sleep on their backs or sides but not on their fronts has been found dramatically to reduce the number of cot deaths.Reuse content