many of the more positive interactions that we would have
had with rogue bacteria. Such interactions help to train the
adaptive immune system, which, after all, was shaped by
natural selection under just these circumstances.
Research shows that while exposure to pathogens (and
the rate of infections) has decreased, rates of both
autoimmune and allergic diseases have increased. The idea
that these two statistics are causally linked is the basis for
the “hygiene hypothesis.” The theory is this: some
infectious agents–notably those that coevolved with us–
protect us from immune-related disorders. Today, the
absence of these pathogens results in weakened immune
systems, leaving them vulnerable to confusion and setting
the stage for type 1 diabetes, multiple sclerosis, celiac
disease, and others.^14
With diabetes, obesity, and even Alzheimer’s disease all
characterized by chronically heightened inflammation–aka
an immune system gone awry–it isn’t a leap to suggest that
our overly sterile lives may be to blame here as well. In fact,
recent research has explored the very link between national
hygiene and Alzheimer’s disease incidence. Using public
sanitation and access to clean drinking water as metrics,
researchers revealed a striking relationship: countries with
greater levels of hygiene had increased incidence of
Alzheimer’s disease, in a perfect linear correlation.
Gluten provides the perfect illustration of how a
confused immune system could lead to an autoimmune