Keeping homes at an almost sterile level of cleanliness and attempting to prevent babies from getting infections could be doing more harm than good and may have contributed to a spike in a type of blood cancer, according to one expert.
Professor Mel Greaves of the Institute of Cancer Research in London has carried out an extensive review of studies and past research on acute lymphoblastic leukaemia (ALL) dating back 30 years.
Writing in the journal Nature Reviews Cancer, he claims that the cancer is caused by a combination of genetic mutations that make people vulnerable and a lack of infection in childhood.
Professor Greaves explained that one in 20 children is born with a genetic mutation that puts them at higher risk of ALL, but only around one per cent of these go on to develop it because their immune system is properly established by coming into contact with bacteria or viruses.
On the other hand, children with the mutation who have not had an early immune system challenge to deal with and then encounter an illness later in life may be more likely to develop a second mutation that eventually leads to the blood cancer.
Professor Greaves said experiments in mouse models that modified the rodents to have the first gene mutation and then kept them in sterile conditions before putting them in a dirty environment saw them develop ALL.
In the 1950s and 1960s ALL was deadly, but although it affects one in 2,000 children today, around 90 per cent will be cured.
However, unlike most illnesses and diseases, ALL is actually increasing in more developed and affluent nations.
"Infectious disease tracks with poverty. The problem is not infection. The problem is lack of infection," said Professor Greaves.
In the least affluent countries where families have lots of children and infection with minor illnesses is common, there are lower rates of ALL.
The expert said he hopes that his discovery could eventually lead to a drug that prevents leukaemia altogether, or is better able to treat it than traditional methods like chemotherapy.
In the meantime, Professor Greaves suggested that parents should ensure their children have plenty of social contact with other infants, perhaps in nursery settings, and do not attempt to keep their homes sterile.
He also recommended breastfeeding and plenty of outdoor play to boost the immune system.
Cancer charities have called the news 'exciting', but added that parents of children with ALL should not feel as though their environment may be at fault.
Only around 650 people each year are diagnosed with ALL each year in the UK, half of whom are children.