Many experts advise common sense. "We don't want to say to children, 'OK, play by the dirty river bank and catch whatever you can, but we can say there's nothing wrong with kids playing in the dirt. They don't have to live in total sanitation, and they won't die from eating something off the floor. It's probably more healthy than not."
On a small family farm in Mongolia, a rooster struts around little Bayar's bed, a goat drinks from his bathwater and livestock serve as babysitters.
By contrast, Mari, growing up in high-rise, high-tech Tokyo, and Hattie, whose doting parents live a "green" lifestyle in San Francisco, both have modern conveniences and sanitation.
Statistically, Mari and Hattie are healthier. Some 42 out of 1,000 children in Namibia, and 41 out of 1,000 in Mongolia die before their 5th birthday; compared with only 8 in 1,000 in the U.S. and only 4 in Japan.
Yet the upscale urban infants are at higher risk for some health problems -- including allergies, asthma and autoimmune diseases like Type 1 diabetes, multiple sclerosis and inflammatory bowel disease -- than the babies in the rural developing world.
Now, there's research that suggests there may be a way to get the best of both worlds.
According to the "hygiene hypothesis," first proposed in 1989, exposure to a variety of bacteria, viruses and parasitic worms early in life helps prime a child's immune system, much like sensory experiences program his brain. Without such early instruction, the immune system may go haywire and overreact with allergies to foods, pollen and pet dander or turn on the body's own tissue, setting off autoimmune disorders.
依据1989年初次提出的“卫生假说” (hygiene hypothesis)理论，孩子在生长早期假如接触多种病毒、细菌和寄生虫，将有利于其免疫系统的展开，似乎这样能促进大脑对其做出更好的防御准备。反之，假如缺乏这类早期接触，免疫系统就可能呈现紊乱，做出过激反响，如对食物、花粉和宠物毛屑过敏等，或者引发人体机理问题，呈现自身免疫失调。
Many of these microorganisms evolved symbiotically with humans over millions of years--the so-called "old friends" theory. But where they've been eradicated, a key part of human development has been thrown off.
"The vast majority of microbes are harmless. There are only a few dozen that can cause lethal infections," says Thomas McDade, director of the Laboratory for Human Biology Research at Northwestern University.
There are other dangers lurking in muddy water and animal feces. Nearly 70% of the 8.8 million deaths of children under age 5 world-wide in 2008 were caused by infectious diseases, most frequently pneumonia, diarrhea and malaria, according to an analysis in the Lancet last week.
Scientists are still working on ways to separate good germs from bad ones; in the meantime, they have a few insights: Studies have shown that children who grow up with household pets have fewer allergies and less asthma than those who don't.