there is less breast feeding of infants and a tendency for them to have fewer social contacts with other children. Both trends reduce babies’ contact with germs. This has benefits – but also comes with side effects. Because young children are not being exposed to bugs and infections as they once were, their immune systems are not being properly primed. “When such a baby is eventually exposed to common infections, his or her unprimed immune system reacts in a grossly abnormal way,” says Greaves. “It over-reacts and triggers chronic inflammation.” As this inflammation progresses, chemicals called cytokines are released into the blood and these can trigger a second mutation that results in leukaemia in children carrying the first mutation. “The disease needs two hits to get going,” Greaves explains. “The second comes from the chronic inflammation set off by an unprimed immune system.” In other words, a susceptible child suffers chronic inflammation that is linked to modern super-clean homes and this inflammation changes his or her susceptibility to leukaemia so that it is transformed into the full-blown condition.