Too much cleanliness can be bad for your baby—so goes the prevailing theory that hypersanitized childhoods may be partly responsible for allergies, asthma, and other diseases. The idea is that early exposures to germs teach an infant's immune system to regulate itself. Just as babies' brains need input, stimulation, and training, so, too, do their immune systems.
But if bugs and viruses are a form of education, which ones make up the perfect curriculum? The research doesn't serve up a neat answer, and, of course, pathogens that actually make kids sick come at a cost. Still, evidence suggests that some gastrointestinal bugs and viruses, which might or might not cause illness, may protect later against allergy, asthma, and inflammation. Baby respiratory infections, on the other hand, probably don't shield kids in the same way. So what's a tiny baby to do? Chew toys off the floor, play in the mud, go to the petting zoo. But stay away from the flu.
The idea that germs protect against allergies started to gain traction around 20 years ago. A researcher named David Strachan found that children with more siblings, particularly older brothers, were less likely to develop hay fever. Strachan's work (like most of the research that followed) didn't prove a causal relationship. Nor did it address how, exactly, kids might school one another's immune systems. But it spurred the theory that all manner of germiness, from dirty hands to runny noses, might help kids in the long run. Researchers also linked growing up on a farm to lower risk of allergy. Ditto for attending day care early on. (Though with caveats. In one study, for instance, day care only seems to protect allergy-prone kids if they attend before they're 3 months old.) But what is it about farms or day care that might help train the budding immune system—the scat, the snot, or something else?
It could be the scat, at least in part. Last fall, researchers analyzed a treasure trove of data from the Philippines, which tracked kids starting when they were in utero, in the 1980s. The data included information on the households the kids were born into as well as the sicknesses and symptoms their mothers reported them having before age 2. The researchers found that kids who were exposed to more animal feces, and who had more diarrhea before they turned 2, tended in their early 20s to have lower levels of C-reactive protein, a key marker of inflammation. This could mean that they had less of the chronic inflammation associated with a host of ills, from rheumatoid arthritis to heart disease, and thus better immune regulation, says anthropologist Thom McDade of Northwestern University, who led the work.
Bugs and viruses that go for the gut also turn up in studies that show lower risk of allergic conditions and asthma. Helicobacter pylori is a bacterium that lives in the stomach lining of as many as half the world's people, often without symptoms, though it's also associated with ulcers. In one paper, preschoolers who tested positive for H. pylori were less likely to suffer from the itchy skin disorder atopic eczema, a hypersensitivity reaction similar to an allergy. In another, H. pylori colonization was linked to a lower risk of childhood asthma.
Hepatitis A, a virus transmitted by contaminated food and water, seems to bolster immune training, too: Kids with a certain common gene variant who had been exposed to hepatitis A appeared to be less likely to suffer from a range of allergic disorders, according to this review by Graham Rook at University College London. (Hepatitis A seems to do this by tweaking the balance of different immune cell types.)
The silver lining of protection against later asthma or allergic conditions is harder to spot for respiratory infections. Papers that sort through the evidence generally find scant evidence that runny noses and sore throats help kids stay healthy later. In fact, children hospitalized for severe respiratory syncytial virus or bronchiolitis may be more likely to develop asthma later on according to Anne Wright of the Arizona Respiratory Center. The flu, too, might spur asthma's development. And early bronchitis or frequent common colds seemed not to lower the risk of atopic eczema —bronchitis, in fact, seemed to increase it. The theme here seems to be: Ingest; don't inhale.
All of this makes some sense in evolutionary terms. Some of today's bugs and viruses have colonized and infected our ancestors, including other mammals, since way back when. In certain cases, we might have evolved in response to their presence. And so these organisms may now help to establish or maintain an aspect of our normal immune regulation. Respiratory viruses probably didn't play this role, Rook says, because they were sporadic and transient, present in some groups of humans but not others. Viral infections like measles, mumps, rubella, or chickenpox probably didn't either, for the same reasons. But pathogens like H. pylori and hepatitis A that infect the gut and are thought to be very old make sense as regulators of immune development. So do microbes found in mud, soil, and rotting vegetation. And so do little worms called helminths.
Note, however, that this is maddeningly hard research to do. Scientists must figure out which critters to pay attention to and then untangle how exactly these organisms interact with the immune system. And they must sort through other factors that probably affect how a child responds to the germy stew of life—like how old he is when exposed, what other infections have already occurred, and what his genetic predispositions are.
And so we're not likely anytime soon to have anything like a lesson plan for boosting your child's immune system by, say, exposing him to H. pylori at 1 month and parasitic worms at 3 months (especially given the risks of deliberate infection). But what we do know helps explain why that hour in the garden or cuddle with the dog is probably all for the good.