Getting Dirty for Good Health
According to this biologist, exposure to a diversity of wild organisms may be essential to ward off illness
THERE'S A FOREST NOT FAR FROM MY HOME where I like to take my kids. They swing on vines, balance on fallen trees and turn rocks. They look, prod and then gather or catch much of what they find. When they’re done, we all sit down in a patch of grass on top of a hill and watch clouds drift by. During these times, I hope that my kids are getting covered by bacteria and other microorganisms.
For this approach, some might consider me a bad parent. After all, as far back as the 1800s, Louis Pasteur and other scientists developed what would later be called the germ theory of disease: the then-revolutionary idea that microbes, by invading our bodies, are the cause of many human diseases. Since then, this theory—by spawning behaviors such as hand washing and drugs that now cure many illnesses—has saved millions, perhaps billions, of lives.
In recent years, however, I’ve come to believe that the germ theory leaves out a lot. My new thinking stems from the findings of scientists working in several different fields of biology. Independently, these biologists have all come to a similar conclusion: Many animal species, even those we tend to think of as dangerous or disgusting, are beneficial to our health.
Some discoveries are old. A small cadre of scientists has long known that many microorganisms that inhabit our bodies and our homes are necessary for a healthy life. Get rid of them and we would die. Without microbes, for example, our digestive systems would stop functioning.
But more recent research suggests that microbes also are needed for the immune system to operate properly. In a number of studies going back to the 1980s, microbiologists and public health workers have found that rural children are less likely to develop allergies than are urban children, a discovery that has been attributed to the fact that country kids, through frequent contact with soil and animals, are exposed to a larger number and diversity of bacteria and other microbes than are city kids. Known as the hygiene hypothesis, the idea is that exposure to microbes is necessary for a child’s immune system to develop. Take away some of the bugs, or even change their composition, and a host of allergic and autoimmune disorders seem to become more common.
Another line of research has spawned the worm hypothesis, which suggests that because our bodies evolved with parasitic worms, the absence of these worms may cause the immune system to overreact, leading to autoimmune disorders such as asthma and Crohn’s disease. The nature-deficit hypothesis, meanwhile, maintains that being outside in a natural setting provides psychological benefits. Without routine exposure to nature and living things, children, especially, develop behavioral, intellectual and other problems.
Taken together, such hypotheses have led me to propose what I call the ecological theory of disease: the idea that illness can arise either from the presence of species that negatively affect our health or from the absence of species that positively affect it. This notion is not new to ecologists. The natural world is filled with examples of plant and animal species that cannot live without other species, from bean plants, figs and corals to termites and leaf-cutter ants.
Thinking Like an Ecologist
Yet physicians, medical researchers and public health officials do not think like ecologists. Even if they did, there is another problem: While we have spent much of the past century studying and trying to control species that are bad for us, scientists have invested little time or effort attempting to identify, much less favor, species that benefit our health.
Take the example of my own home. Recently, as part of my lab’s Your Wild Life project, I attempted to catalog all of the species on my body and in and around my house. I came up with a list of more than 2,000, yet neither I nor any other scientist can identify most of these species. The majority, particularly microbes, do not yet have scientific names. But even insects—species visible to the naked eye as they run across the carpet or hang out on window screens—are more often than not essentially unknown. Which ones should I favor? Nobody has a clue.
Worse yet, look around at the broader world and ask which of the millions of poorly known or unknown species on Earth we should attract and nurture. At current rates, it will take our poorly paid and unappreciated taxonomists hundreds of years at least to even name all of the life-forms around us.
Meanwhile, like everyone else, I am left to figure out on my own how to stay healthy and, more importantly, how to keep my children healthy. As a scientist, I work to identify and study the diversity of species on the planet. But as a father, I am left to more primitive magic: I take my kids to the hill near our house, where we bury our fingers in mud and leaves in the hope that the good life will creep up on us.
How to Get The Dirt on Dirt
When we let children play in dirt, we're not just allowing them to explore nature, we’re exposing them to healthy bacteria and other microbes that create stronger immune systems. Contact with dirt also may reduce stress.
A recent NWF report, The Dirt on Dirt, details how getting dirty helps kids lead happier, healthier lives.
Rob Dunn is a biologist and science writer at North Carolina State University in Raleigh. His latest book, published in 2011, is The Wild Life of Our Bodies.