Acute lymphoblastic leukaemia (ALL) is the most common type of childhood cancer, affecting one in 2,000 children. It is particularly prevalent in advanced, affluent societies and figures show it is increasing globally at a rate of around 1% per year.

Now, a major new analysis has identified the most likely cause of most cases and suggests that the disease may be preventable.

Professor Mel Greaves from the Institute of Cancer Research examined more than 30 years of research into the genetics, cell biology, immunology, epidemiology and animal modelling of childhood leukaemia.

He concluded that the disease is caused through a two-step process of genetic mutation and exposure to infection.

The first step involves a genetic mutation that occurs before birth and predisposes children to leukaemia. However, only 1% of children born with this genetic change go on to develop the disease.

Next, a lack of exposure to benign bacteria or viruses in the first year of life can mean that the immune system is not set up to deal with threats correctly.

The child may then later encounter an infection such as a cold or flu and go on to develop a second genetic mutation that will make them susceptible to the cancer.

Professor Greaves is now investigating whether earlier exposure to harmless 'bugs' could prevent leukaemia in mice. If so, it raises the possibility that it could be prevented in children through measures to expose them to common but benign microbes to help train the immune system.

Commenting on the study, which has been published in Nature Reviews Cancer, Professor Greaves said: "The research strongly suggests that ALL has a clear biological cause, and is triggered by a variety of infections in predisposed children whose immune systems have not been properly primed.

"It also busts some persistent myths about the causes of leukaemia, such as the damaging but unsubstantiated claims that the disease is commonly caused by exposure to electro-magnetic waves or pollution.

"I hope this research will have a real impact on the lives of children. The most important implication is that most cases of childhood leukaemia are likely to be preventable.

"It might be done in the same way that is currently under consideration for autoimmune disease or allergies -- perhaps with simple and safe interventions to expose infants to a variety of common and harmless 'bugs'."

Factors that may decrease a baby's risk include going to a day care centre and having older siblings, which exposes them to bacteria, as well as breastfeeding and being born vaginally rather than by caesarean section.

It might also be possible to give children a safe dose of bacteria, such as in a yoghurt drink, to help prime their immune system.

Professor Paul Workman, chief executive of the ICR, said: "It's exciting to think that in [the] future, childhood leukaemia could become a preventable disease as a result of this work. Preventing childhood leukaemia would have a huge impact on the lives of children and their families in the UK and across the globe."