After eight hours in an overheated Soviet-era sleeper car, we pull into the Petrozavodsk train station just after 1 a.m. The streets are silent, the night air chilly. Our taxi shudders and swerves along roads pitted with axle-gulping potholes. Identical concrete apartment blocks built in the 1960s flash by in a blur. Winter temperatures here, some 250 miles northeast of St. Petersburg, sometimes plunge to minus 40 degrees Fahrenheit. A traffic circle in the middle of town boasts what locals claim is Russia’s only statue of Lenin holding a fur hat.

I’m traveling with Mikael Knip, a short, energetic Finnish physician and University of Helsinki researcher with a perpetual smile under his bushy mustache. He has come to Petrozavodsk—an impoverished Russian city of 270,000 on the shores of Lake Onega and the capital of the Republic of Karelia—to solve a medical mystery, and perhaps help explain a scourge increasingly afflicting the developed world, the United States included.

For reasons that no one has been able to identify, Finland has the world’s highest rate of Type 1 diabetes among children. Out of every 100,000 Finnish kids, 64 are diagnosed annually with the disease, in which the body’s immune system declares war on the cells that produce insulin. Type 1 diabetes is usually diagnosed in children, adolescents and young adults.

The disease rate wasn’t always so high. In the 1950s, Finland had less than a quarter of the Type 1 diabetes it has today. Over the past half-century, much of the industrialized world has also seen a proliferation of the once rare disease, along with other autoimmune disorders such as rheumatoid arthritis and celiac disease. Meanwhile, such afflictions remain relatively rare in poorer, less-developed nations.

Why?

Petrozavodsk, only about 175 miles from the Finland border, may be the perfect place to investigate the question: The rate of childhood Type 1 diabetes in Russian Karelia is one-sixth that of Finland. That stark difference intrigues Knip and others because the two populations for the most part are genetically similar, even sharing risk factors for Type 1 diabetes. They also live in the same subarctic environment of pine forests and pristine lakes, dark, bitter winters and long summer days. Still, the 500-mile boundary between Finland and this Russian republic marks one of the steepest standard-of-living gradients in the world: Finns are seven times richer than their neighbors across the border. “The difference is even greater than between Mexico and the U.S.,” Knip tells me.

Since 2008, Knip and his colleagues have collected tens of thousands of tissue samples from babies and young children in Russia and Finland, as well as in nearby Estonia. In his spotless lab on the fourth floor of a modern research complex in Helsinki, nearly two dozen freezers are filled with bar-coded vials of, among other things, umbilical cord blood, stool samples and nasal swabs. The freezers also hold tap water and dust collected at the different locations. By comparing the samples, Knip hopes to isolate what’s driving Finland’s diabetes rate up—or what’s keeping Russian Karelia’s low.

For all the sophisticated analysis involved, the theory that Knip is testing couldn’t be more basic. He thinks the key difference between the two populations is...dirt. In a sense, he wonders if kids in Finland, and in the United States and other developed nations as well, are too clean for their own good.

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The idea that dirt, or the lack of it, might play a role in autoimmune disease and allergy gained support along another border. In the late 1980s, Erika von Mutius was studying asthma in and around Munich. At the time, researchers thought air pollution was the cause. But after years of work, the young German researcher couldn’t clearly link Munich’s pollution and respiratory disease.