Kids home from school, empty cubicles at work… Why all the coughing and sneezing this fall, even before the really cold, wintery time of year? The timing relates to kids having gone back to school, where germs are passed easily among kids and then on to their working parents, who then import them to the office, laying the groundwork for germ transportation all winter long. It’s enough to make you want to hole up in a Cloroxed linoleum closet with a big bottle of Cipro and a fire extinguisher full of Purell.
But what if living in a superclean environment actually makes you sick, or increases your risk of cancer? It's now clear that highly hygienic environments, especially in infancy, play an important role in the skyrocketing occurrence of asthma, allergies and autoimmune disease.
The "hygiene hypothesis" was first raised in 1989, when British doctors noticed a lower rate of hayfever among children who had more siblings. Later studies showed reduced risk of asthma, allergies and autoimmune diseases among children with more intense or richer exposures to microbes, including larger family size, time spent near horses, livestock, or stables, and number of household pets. Rates of allergy and leukemia are 30-50 percent lower among children who attend daycare than those who don’t. And more recent data suggest that celiac disease is more common among children born in the summer, when needed exposure to seasonal germs might be lowest.
Ongoing research being conducted at Stanford and the affiliated Cancer Prevention Institute of California is focused on whether these kinds of associations also hold for adult diseases linked to chronic inflammation, not just allergic and autoimmune conditions but deadly breast and colon cancers.
Modern urban life radically reduces exposure to microbes and parasites that have been part of the human ecosystem for eons. These microbes not only include the bacteria and viruses that make you sick, but also those that don’t, including friendly bacteria like lactobacillus that live in your gut, and other benign microbes that live in dirt and untreated water. Parts of dead microbes are probably also important. The immune system is known to be stimulated by the inhalation of bacterial cell wall components called "endotoxin" that become airborne as cow manure or dog poo dries up. Exposure to cow manure may explain why dairy farmers have substantially lower rates of lung cancer despite smoking.
It is biologically reasonable that microbial exposures might influence cancer and inflammatory diseases. In evolutionary terms, the removal of many of these microbes from daily life in the last two generations is very sudden. It is reasonable that babies’ new immune systems may need these microbes to calibrate themselves, so as to respond with the right firepower for the threat at hand. Without adequate calibration, the immune system may overreact to normally safe substances, like pollen, dog fur, or peanuts, or get stuck in a chronic state of overreaction, causing inflammation. It is also likely that under-exposure to microbes skews gut bacterial ecosystems to create inflammatory immune responses.
Researchers are starting studies to explore how probiotics, or other controlled exposures to microbes, might be new tools for preventing cancer. As this and other studies to understand the lifelong health consequences of the hygiene hypothesis continue, it is unlikely that modern lifestyle preferences will revert back to the muddy and germy. Perhaps there will be evidence someday for controlled exposures to immune-boosting probiotics or benign microbes. But in the meantime, don’t necessarily curse the coughing co-worker or sneezing schoolmate - they may actually be lowering cancer risks for you and your children.
Christina Clarke, PhD, MPH, is a research scientist at the Cancer Prevention Institute of California *CPIC) and a member of the Stanford Cancer Institute. Part of the Stanford Cancer Institute, the Cancer Prevention Institute of California conducts population-based research to prevent cancer and reduce its burden where it cannot yet be prevented.
Previously: Eat a germ, fight an allergy
