Newcastle University led study about the risk factors of childhood cancer and came to some interesting conclusions.

Apparently, leukemia and brain tumors may be caused by relatively minor ailments like a respiratory virus or even a mild flu.
This finding does not apply to all children as a rule but rather to children that are genetically suspectable to cancer. Children who already carry the mutant cells for cancer can be easily affected as the flu or cold virus could strike the mutant cell and give rise to a second mutation.

Other possible risk factors have been found such as an infection suffered by the mother during pregnancy or geographical locations and environmental factors.
More evidence is needed to support these findings but scientists are certainly hoping for the development a vaccine that would serve to protect children.