Epidemiological studies report opposing influences of infection on childhood B cell acute lymphoblastic leukemia (B-ALL). Although infections in the first year of life appear to exert the largest impact on leukemia risk, the effect of early pathogen exposure on the fetal preleukemia cells (PLC) that lead to B-ALL has yet to be reported. Using cytomegalovirus as a model early-life infection, we show that virus exposure within one week of birth induces profound depletion of transplanted …