Scientists have shown how common infections might trigger childhood leukaemia.They have identified a molecule, TGF, produced by the body in response to infection that stimulates development of the disease.It triggers multiplication of pre-cancerous stem cells at the expense of healthy counterparts.The Institute of Cancer Research study appears in the Journal of Clinical Investigation.Leukaemia occurs when large numbers of white blood cells take over the bone marrow, leaving the body unable to produce enough normal blood cells.The researchers had already identified a genetic mutation - a fusion of two genes - occurring in the womb that creates pre-leukaemic cells.These cells then grow in the bone marrow, effectively acting as a silent time bomb that can stay in the body for up to 15 years.Evidence suggests the mutation may be present in as many as one in 100 newborn babies, but only about one in 100 of these children then go on to develop leukaemia.This suggests that the cells will only complete the transformation to fully-fledged cancer cells if they exposed to an independent trigger.The latest study suggests production of TGF in response to an infection could be that trigger.Because the molecule hugely increases the rate at which the pre-leukaemic cells multiply, this significantly raises the the chance that some will become even further damaged in a way that results in the child developing leukaemia.Preventative measuresResearcher Professor Mel Greaves said: "Identifying this step means we can determine how an unusual immune response to infection may trigger the development of the full leukaemia and eventually perhaps develop preventative measures such as a vaccine."Dr Shabih Syed, scientific director at the charity Leukaemia Research, said: "Before this study, there had been only circumstantial evidence to implicate infections in the progression from a child carrying pre-leukaemic cells to actually having leukaemia."There was no evidence of the mechanism by which this might happen."While infection is clearly only one factor in triggering progression, this study greatly increases the strength of evidence for its role in the commonest form of childhood leukaemia."Source: BBC