Delayed exposure to infection may trigger leukaemia after prenatal damage to DNA

About one child in 2000 develops leukaemia before the age of 15: most cases are acute lymphoblastic leukaemia.1 Childhood leukaemia is a biologically diverse disease, so several pathways to its development are possible. All probably combine genetic susceptibility and exposure to external risk factors at a time when the child is vulnerable.

Children with Down's syndrome and certain other genetic syndromes are much more susceptible to leukaemia, but such conditions are involved in relatively few cases.2 Bigger babies3 and boys1 have a slightly higher risk.

Many external factors have been reported as associated with an increased risk of childhood leukaemia.4 Some may be causal, but some may merely be correlated with the actual cause. Other apparent associations may be due to chance or bias: researchers doing case-control studies often have difficulty selecting controls who are representative of the population. Factors are more likely to be causal if they are biologically plausible, are seen consistently in different populations, and have a dose-response relation to the risk of leukaemia.

In the 1950s a very large case-control study of childhood cancer in the United Kingdom found that radiography of a mother's abdomen during pregnancy increased her baby's risk of developing childhood leukaemia by …