Because each of these factors has been associated with reduced risk of allergies and related diseases, researchers can't pinpoint which factor or combination of factors provides the protection.
Parasites and disease-causing microbes have also shown a protective effect, but again it's not clear which microbes are doing the protecting. A 2007 study that compared genetically related children living in Finland and Russia found that the Russian children — who were less wealthy than their Finnish counterparts and who more frequently showed antibodies to the hepatitis A virus, H. pylori bacteria and other microbes associated with poor hygiene practices — were far less likely to have allergies. The findings made it clear that microbial infections and environmental differences were conferring an advantage, but they were less conclusive about which infections conferred the greatest advantage.
So what does all this mean? Should we ditch spring cleaning and adopt a dairy cow — or a parasite — to keep allergies at bay?
Probably not, says Barnes: Modern hygiene saves lives and prevents the spread of disease, and no researcher would advocate abandoning it entirely. But we may want to rethink our relationship with germs, she says.
"Knowing what I know about the hygiene hypothesis, I think twice before I run to a physician for an antibiotic," she says. "I also think about the foods my family eats. We eat a lot of yogurt for the beneficial bacterial cultures it provides."
Zasloff goes even further. He doesn't mind if his kids eat a little dirt, don't wash their hands before every meal or wear the same socks twice. Eating food that's been in the fridge a while or that has fallen on the floor is okay, too, he says.
That may not be for you. The important thing, Zasloff says, is moderation: "It's not that you should expose yourself to things that are going to kill you. We're just talking about living in a more microbially rich environment. That means you don't need to use antibacterial soaps or wipes, or clean everything with bleach, or even wash your clothes every day. Getting dirty isn't so bad. . . . Just use your common sense."