Dear Science,

Recently, a friend who is in public health told me about the hygiene hypothesis: One in five Americans has an allergy, while the number in third world countries tends to be far lower—possibly due to our excessive cleanliness. Should I be taking a daily dose of mud with my multivitamin? Does the hygiene hypothesis have any scientific merit, or is it just the next fad for people who don't vaccinate their kids?

Baking Dirt Pies

The hygiene hypothesis has a neat symmetry to it: Our immune systems, bored by our crisply clean modern environment, are battling false enemies—random junk floating around (allergies) and our own bodies (autoimmune diseases). The correlation that you noted—places with tons of infectious diseases have comparatively low rates of allergies and autoimmune disease—holds true even as people move around the planet. Southeast Asians in Southeast Asia are much less likely to have allergies than Southeast Asians living in the United States.

Animal studies provide hints of how this might work—there is evidence of classes of immune cells left fallow too long suddenly lashing out against something, anything. It all makes a bit of sense. Human beings have one of the strongest and most violent immune systems of any living thing. Blame it on selection bias. We're the survivors of filthy cities, or the descendants of people who lived for generations in filthy cities. Smallpox, syphilis, plague, measles, mumps, rubella, pertussis, tuberculosis, worms of all shapes and sizes, malaria—if you're alive today, some miserable ancestor of yours from merely one or two hundred years ago had to survive an onslaught of these. This is no small feat.

With all this said, the hygiene hypothesis still isn't completely proven. For humans at least, it remains a correlation rather than demonstrated cause and effect. We still don't quite know which sorts of infections, if any, would be the right ones. Being continuously infected with worms, viruses, bacteria, and tuberculosis isn't so great for health, either—allergy-free or not. Scientists right now are trying inoculations with gentle bacteria (probiotics), purified parts of these pathogens, and even gentle stimulators of the fallow immune cells, all as a way to get the possible benefit of being exposed to disease without the downsides of having, say, malaria swimming in your blood. As you can imagine, these are slow and careful studies, done on otherwise healthy people. Nobody wants to make people ill in the effort to demonstrate some protection from controlled exposure to filth.

So, for now, let your kids play in dirt. But vaccinate them. What is vaccination, other than safe, controlled exposure to some of these pathogens? If you believe this hygiene hypothesis argument, vaccination matters even more.

Cleanly Yours,


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