Humans evolved in a dirty environment. We have been on the planet for hundreds of thousands of years but soap, antiseptics, disinfectants, canned and frozen foods have only been with us for a few generations, and antibiotics arrived less than a century ago.

During most of our history our environment was replete with bacterial and viral hazards, and our immune systems were constantly challenged.

One of the everyday challenges was from a molecule called beta 1,3-1,6 glucans present in yeasts and mould, which the innate immune system came to depend on to operate at full efficiency – see right panel. 

Reduced response to environmental threats

In today’s over-sanitised environment our immune systems have relatively little to contend with, leaving them less active and less able to neutralise new and unexpected threats.

This explains why, when we travel to parts of the world where sanitation standards are lower than ours, we routinely fall victim to pathogens the locals have no problems with.  Our vulnerability to ‘Montezuma’s revenge’, other travellers’ ills and sudden infection is largely due to our under-strength immune systems.

And rapid global travel means that on a single day, a single passenger can contract a virus or bacterium on one continent and arrive in another before the first symptoms of illness emerge, thus avoiding early detection.

Epidemic of asthma, allergy and cancer

The so-called ‘hygiene hypothesis’ has left our immune systems off-balance and also more prone to react to normally harmless substances such as pollen. This has contributed to the current epidemic of asthma and allergy.

The weakening of the innate immune system is also implicated in the huge increase in cancer that has occurred in the last half-century.