It was a revelation. Germs cause disease. When Louis Pasteur and Robert Koch discovered and developed what would later be called the germ theory in the 1860s, this was a radical, then revolutionary idea—one so good it seems obvious in retrospect.
At the heart of their work was the notion that individual species cause disease by invading our bodies. Over the next century, the notion of "germs" changed our behavior. It led us to scrub our hands and actively fight specific pathogens (as researchers came to call dangerous germs) and to cure the diseases they cause. These changes saved millions, maybe billions of lives. Every day you rub shoulders with the success of this theory. How could there be anything wrong with it?
New research, however, is beginning to question, if not germ theory itself, at least some of the actions we have taken on its behalf. These studies come from very different groups of scientists, largely working separately and apparently without much awareness of one another. But I believe that they are unwittingly part of the slow unraveling of a new, broader theory of disease, the ecological theory of disease.
Here's the thinking. In the late 1980s, microbiologists and public-health researchers began to notice differences between rural and urban kids. Rural kids seemed less likely to develop allergies. A new idea was floated—perhaps they had been exposed to more bacteria that had helped their immune systems to "balance" themselves. This idea, often called the hygiene hypothesis, has since found support in empirical studies worldwide.
Country kids whose fingers still plunge regularly into the rich bacteria of soil (and farm animals) have fewer allergies. But it isn't just farm living: Sometimes the exposure to a wilder bacterial life can be subtle. For example, a recent study in Australia found that pregnant mothers living with dogs were less likely to have children with allergies. These studies note fundamental differences between the immune systems of dirty kids and clean kids. Conclusion: In some ways it is better to be dirty.
More recently, a new version of the hygiene hypothesis has suggested that it isn't just large numbers of bacteria that it is good to be exposed to but, rather, many kinds of bacteria. Our immune system needs to be exposed to many species in order to sort the good from the bad. Without such exposure, argues this "biodiversity" version, mistakes get made. The immune system, in not having seen enough of the world, doesn't know quite what to attack. It attacks pollen. It attacks us.
This made me sit up and take notice. There are, I realized, many separate fields of science in which the failure to be exposed to good species or even just a diversity of species is believed to make us sick.
The "worm hypothesis" argues that our bodies evolved with parasitic worms as a dependable presence, and that for some individuals the absence of such worms causes the immune system to overreact, leading to autoimmune diseases such as Crohn's, multiple sclerosis, and asthma. The nature deficit hypothesis, on the other hand, argues that lack of exposure to nature in our city environments causes psychological problems in children who then suffer from any of a variety of behavioral and other problems. This is country cousin to the biophilia hypothesis, which suggests an innate fondness for nature and biodiversity, which both bring us benefits and, in their absence, costs.
All of these relate to the much older and well-accepted "deficiency" model, which correctly states that diseases such as scurvy are caused by the absence of whole classes of species (and their nutrients) in our diets.
What seems to have gone pretty much unremarked is that these ideas all suggest ways in which the absence of beneficial or historically common species in our lives can make us sick. In a way, taken together these ideas make up the obverse of the germ theory of disease; if the germ theory is about bad species being present, these hypotheses are all about good species that have gone missing.