Dear Julia: I've read that being too clean can actually make you less healthy. Is that true? Can extra exposure to dirt and germs actually be good for you?

My father rarely gets sick. I can count on one hand the number of times he's come down with so much as the sniffles. He attributes this robustness to his childhood on a farm in the foothills of Northern Italy. "I grew up around lots of cow and pig shit," he'll brag. "Why do you think I'm built like this?"

In his own colorful way, he's referring to an idea known as the "hygiene hypothesis." First described in 1989 by David Strachan, a London-based epidemiologist, the idea is that people exposed to a diversity of microorganisms early in life lower their risk of allergic diseases like asthma, eczema, seasonal allergies, and even autoimmune disorders such as arthritis or inflammatory bowel disease.

The logic of the hygiene hypothesis is pretty alluring: "A child's immune system needs education," Erika von Mutius, a pediatric allergist at the University of Munich, recently told my colleague Joseph Stromberg. "The hygiene hypothesis suggests that early life exposure to microbes helps in the education of an infant's developing immune system." That's why places like farms, or even houses with pets, are thought to have some protective effect against allergies for kids.

That said, scientists haven't yet proved the hygiene hypothesis. It's still just that — a hypothesis. And even if it's true, it's a lot less sweeping than a lot of people tend to think.

The hygiene hypothesis was originally based on data showing that British children with one or more older siblings had a lower incidence of hay fever. More kids in a house presumably meant more microbes, and hence more robust immune systems. Some researchers thought the hypothesis might also explain why allergies and other autoimmune diseases became more common in the 20th century — perhaps it was because we were all becoming more obsessed with hygiene and cleanliness.

But since then, a lot more work has been done on the hypothesis, and contradictory evidence has emerged. There's research that suggests having more siblings seems to be associated with higher rates of asthma, not less. What's more, there are lots and lots of possible reasons allergies and other autoimmune diseases might have been rising in the 20th century including changes in food production, agricultural practices, and diet; the eradication of many parasitic worm infections; and the introduction and proliferation of home heating and ventilation systems.

For these reasons, the researchers at UpToDate, a website that compiles medical evidence for doctors, concludes: "At this point, we have only hypotheses, with evidence to both support and refute them, not definitive conclusions."

Also keep in mind that the hygiene hypothesis mainly focuses on specific ailments — allergies and autoimmune disorders — not infectious diseases like cold or the flu. While the hypothesis has led to compelling studies on things like the relationship between C-sections, probiotics, and allergic diseases, the researchers I spoke to were hesitant to give out any health advice based on the idea that more dirt might be good for you.

Robert Wood, an immunologist at Johns Hopkins, pointed out that much of the research is epidemiological, looking at large populations, which makes the findings difficult to apply to individuals. "We have no idea how dirty someone should keep a household to provide protection," he said.

Von Mutius also pointed out to me that the hygiene hypothesis tends to focus on the benefits of particular microbes found in specific environments — say, how growing up near barnyard animals or in the inner city might affect allergy rates. The hypothesis says nothing about personal hygiene. So no one is suggesting that if you stop washing your hands, you'll somehow be healthier.

Quite the contrary: There's lots of evidence that personal hygiene can help reduce the risk of infection. In one 2008 review of the evidence, researchers found hand washing cuts incidents of diarrhea by about 30 percent. Another 2008 review also found improving hand hygiene reduce the number of gastrointestinal infections by 31 percent and respiratory illness by 21 percent. This is why public health organizations urge people to wash their hands as an effective measure for all sorts of things, from staving off Ebola to stopping the spread of flu.

So that's where we are: Scientists are realizing that some bacteria and microbes definitely have beneficial health impacts. But researchers still aren't certain which bacteria and in which combination help, or how population-wide differences in microbial environments and their related allergy rates apply to individual cases. While they're figuring that out, the evidence suggests that good hygiene and hand washing can go a long way.

Further reading: For more detail, see this Vox article on the hygiene hypothesis, and this one on probiotics, the "friendly bacteria."

Welcome to Dear Julia, a weekly column where readers can submit everyday health questions. Which over-the-counter painkillers work best? Is it better to run or walk for exercise? How much harm does frequent flying do to your body? Julia Belluz will sift through the research and consult with experts in the field to figure out how science can help us live happier and healthier lives.

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