Sending your baby to day care in the first few months of life could protect them against leukaemia, say UK experts.
Exposure to infections is good for infants, say doctors
The Leukaemia Research Fund team believe exposure to common infections in early infancy is good and helps "prime" the immune system.
Conversely, reduced exposure to bugs in the first year of life increases the risk of developing acute lymphoblastic leukaemia (ALL), they suggest.
The findings of their 10-year study appear on bmj.com.
Childhood leukaemia - cancer of the blood cells - has been increasing at a rate of about 1% a year.
In children, about 85% of these are acute lymphoblastic leukaemia or ALL and acute myeloid leukaemia (AML) accounts for most of the rest.
There have been many theories about what might trigger leukaemia, including exposure to radiation from the environment.
But the authors of the current study say they now have compelling evidence that exposure to infections in infancy is key.
This notion has been around for decades, but the UK Childhood Cancer Study is one of the largest to look at this link.
It involved 6,305 children aged 2-14 years without cancer and 3,140 children with cancer, of whom 1,286 had ALL.
Parents were asked about day care and social activity with children outside the family during the first year of life.
The researchers found that increasing levels of social activity outside the home were linked to consistent reductions in the risk of ALL.
The greatest reduction in risk - of 52% - was seen in children who attended formal day care on a regular basis during the first three months of life.
Those youngsters exposed to informal day care, such as that provided by friends and family, saw a 38% drop in risk of ALL, while those who had some social activity, but not day care, had a 27% lower risk.
Lead researcher Professor Mel Greaves, of the Institute of Cancer Research in London, said: "There is good biological evidence now that with childhood leukaemia, there is an interesting double whammy.
"First, there is a genetic mutation that occurs in the baby while they are in the womb, which happens very commonly."
Then, in 1% of those children, after birth, there is a trigger that causes a second mutation in the genes, he said.
"There is abundant evidence now that the immune system requires infection in the first few months of life in order to be set up and function normally."
If this does not happen, when the child is older and encounters an infection, that infection can then trigger the leukaemia, he said.
"Infection early in life is good for you, it protects you - pretty much what your grandmother might have told you," he said.
Dr David Grant, scientific director of the Leukaemia Research Fund, which funded the research, said: "This is very reassuring for parents. We have had a lot of theories about what causes leukaemia, but this study takes us to a very firm conclusion.
"The more implausible theories I think we can now dismiss."
He said treatments were improving and that now about 75% of children with leukaemia survive the disease.
"But if we can prevent the disease occurring in the first place, which I think now is quite possible, then this has been a tremendously helpful study," he said.