Too much clean living may be contributing to a surge in cases of Alzheimer's disease in developed countries, experts believe.
Scientists have linked the 'hygiene hypothesis' – the idea that lack of exposure to germs, viruses and parasites harms the immune system – to rising rates of Alzheimer's in richer nations.
Evidence shows that countries where the risk of infection is relatively low have more people suffering from Alzheimer's.
Likewise, better sanitation and the expansion of cities go hand-in-hand with higher incidence of the disease, the most common form of dementia.
Taken together, infection levels, sanitation and urbanisation account for 42.5pc of the variation in rates of Alzheimer's between different countries.
Dr Molly Fox, from Cambridge University, who led the new research published in the journal 'Evolution, Medicine and Public Health', said: "The 'hygiene hypothesis', which suggests a relationship between cleaner environments and a higher risk of certain allergies and autoimmune diseases, is well established.
"We believe we can now add Alzheimer's to this list of diseases. There are important implications for forecasting future global disease burden, especially in developing countries as they increase sanitation."
The scientists looked at the link between hygiene and Alzheimer's rates in 192 rich and poor countries. They adjusted the findings to take account of differences in birth rate, life expectancy and age structure.
Access to clean drinking water was one area said to have a high impact on Alzheimer's rates. Countries such as the UK and France, where this is universal, had a 9pc higher incidence of Alzheimer's than countries such as Kenya and Cambodia where less than half the population can access clean water.
A similar pattern emerged from comparisons between countries with low and high rates of infectious disease.
Super-clean Switzerland and Iceland, with very low rates, were 12pc more affected by Alzheimer's than China and Ghana, whose infection rates are high.
The more urbanised countries also experienced higher rates of Alzheimer's irrespective of life expectancy.