London - Our instinct is to keep babies clean, to scold them if they pick up something dirty and shield them from other sick children - but could this be increasing their risk of cancer?
A new study - the result of analysis of 30 years of data - suggests that a lack of exposure to bugs as a baby could be to blame for the rise in cases of acute lymphoblastic leukaemia (ALL), the most common form of childhood leukaemia.
The study’s author, Professor Mel Greaves, of the Institute of Cancer Research in London, said that an infection could trigger the cancer - but only in genetically-prone children who had a lack of microbial contact within the first year of their life.
In other words, the cancer seemed to strike mainly those who had a "clean" infancy.
Having siblings or going to playgroups, thereby gaining more exposure to a host of microbes, seemed to be protective.
Work is now ongoing to develop a bacteria pill that could be given to children to protect them from cancer and a host of other conditions, from asthma to diabetes, that have been linked to a lack of exposure to bugs.
Which begs the question, are we keeping our children too clean?
The idea that being overly clean might be bad news emerged in the 1980s with the hygiene hypothesis - the suggestion that excess cleanliness underworks our immune system so that it overreacts to harmless substances such as pollen. It was argued that this was why rates of allergy-related conditions have risen.
Now it is thought that it’s not hygiene that is making us ill but modern lifestyles, including our failure to spend time outside.
The argument is that we evolved to live alongside bugs commonly found in the air, water and on animals, and a lack of exposure to these "old friends" is causing inflammation and, ultimately, an increase in autoimmune disorders such as type 1 diabetes.
"The immune system of vertebrates is like a computer, and when you’re born it has hardware and software but no data, so it can’t function until the data is put in it," says Professor Graham Rook, a medical microbiologist at University College London, who developed the "old friends" theory.
Being born by Caesarean, not being breastfed, being the firstborn (so no siblings about), antibiotics and a lack of fibre (a source of food for beneficial gut bacteria) all reduce contact with "old friends", adds Professor Rook.
"But we’re not saying people need to be less hygienic," he stresses. "Years ago, people were dying through lack of good hygiene."
Sally Bloomfield, a hygiene expert and honorary professor at the London School of Hygiene and Tropical Medicine, explains it’s targeted hygiene that is crucial. "So we need to worry not about whether something is dirty, but rather whether it is a risk that could make us ill."
However, some elements of cleanliness are unnecessary for good health for most people, she says.
Bleaching sinks and drains is one. ‘Most people do it to control odours, but when it comes to protecting against infection, if the microbes are down the plug-hole, they’re not likely to be a risk,’ says Professor Bloomfield.
The study found that children whose parents licked their dummies clean were less likely to have eczema at 18 months of age than those whose dummies were washed or sterilised.
One very simple way to improve our exposure to immune-boosting bugs is to get outside more.
"About a third of the organisms in your gut produce spores that can persist in the environment for thousands of years," says Professor Rook. Therefore, where humans have been, "the environment is seeded with strains". So if we go out, we can breathe in beneficial microbes dropped by our forefathers.