It may come as a surprise that Finland - one of the least polluted, wealthiest countries with a high life expectancy - has the highest rate of Type 1 diabetes. Each year, there are 58 cases diagnosed per 100,000 children; in the US there are 24, the International Diabetes Federation says.
Some researchers suspect there may be a connection between Finland's cleanliness and the incidence of the disease. They are now investigating whether the lack of exposure to a specific group of bacteria found in the intestine may be causing weaker immune systems in Finnish children, making them more susceptible to Type 1 diabetes.
This so-called hygiene hypothesis - that cleaner living can result in a weaker immune system - has also been linked to ailments such as asthma.
"We are working along the idea that we have a trigger which most likely is an infectious agent," said Mikael Knip, a professor of pediatrics at the University of Helsinki who has been studying diabetes for 30 years. "There is an association between such infections and appearance of antibodies."
Just as there are microbes that trigger the disease, Knip says there are also some bacterial or viral infections that, if they occur at an early age, can protect a young child from developing Type 1 diabetes.
Type 1 diabetes, which affects about 37 million people worldwide, is an autoimmune disease in which the body does not produce sufficient insulin, a hormone needed to break down sugars. Typically diagnosed in children, teens and young adults, the disease can eventually damage the eyes and organs such as the kidneys, and it increases the likelihood of stroke and heart failure. Type 1 diabetes can shorten a person's life span by as many as 10 years and there is no cure.
Knip and fellow researchers took blood and tissue samples from 3,500 children - from Finland, Estonia and Karelia, a part of Russia that once belonged to Finland. While residents of the three areas share similar language and genes, the rate of Type 1 diabetes is remarkably different, as are their levels of hygiene.
At 58 cases per 100,000, Finland's rate is about six times as high as Karelia's.
Estonia's is between 25 and 30 cases per 100,000, but it has tripled in the past 15 years, catching up with its Western European neighbours. Karelia, just north of St Petersburg, is poorer, and children there grow up in an environment that is dirtier than the two other study areas, according to Knip. There, the rate is less than 10 cases per 100,000 per year.
"We are studying babies and young children to look whether we have a specific microbe of the infections or whether the total microbial load is important," he said.
Microbes found in stool samples from each child and in dust particles from their homes are also being analysed by researchers at the Harvard/MIT Broad Institute and Massachusetts General Hospital in Boston.
"What we have been able to do is get a detailed trajectory of how bugs change and how stable the microbiome is," said Ramnik Xavier, chief of gastroenterology at Massachusetts General and senior associate at the Broad Institute. "The only way is to identify the bugs and then do experiments to see whether they" cause diabetes to develop.
Some scientists are sceptical of the cleanliness-disease connection. "It's plausible, but it's a long reach," said Desmond Schatz, medical director at the University of Florida's Diabetes Centre of Excellence.