You have to eat a peck of dirt before you die. It’s an old adage that’s taking on new relevance.

Many doctors have long argued that a certain level of exposure to germs is good for you. The “hygiene hypothesis” has been around since the late 1980s. But a renewed emphasis on cleanliness, thanks to a string of viral outbreaks (SARS in 2003, bird flu in 2006-07, and H1N1 in 2009), has reinvigorated the debate about whether or not it is possible to be too clean.

Mary Ruebush, an immunologist and the author of Why Dirt Is Good: 5 Ways to Make Germs Your Friends (Kaplan, $22.95), explained the theory to the Georgia Straight.

“What makes your immune response strong is exposure to things in your environment,” she said in a telephone interview from her home in Bozeman, Montana. “So the more you are challenged by things around you, the stronger your immunologic muscles get.”

In the book, Ruebush suggests that exposure to bacteria can be especially important for children whose immune systems are still developing. She maintains that parents should put away the antibacterial soap and let kids play with the dog.

“You’ve got an army of loaded weapons running around your body, and their job is to protect you from the things that could injure you,” Ruebush explained. If you remove all potential threats from your environment, those weapons have a tendency to turn inwards. The result, she said, can be an increased susceptibility to allergic reactions and chronic diseases like asthma.

This is why the world’s most developed nations are seeing steadily increasing rates for these autoimmune diseases, Ruebush argued. Immune systems are increasingly left unprepared and out of practice.

“We’re growing a generation of children who have weakened immune systems, and then we’re growing a generation of bugs that can’t be killed,” she said. “It is exactly the opposite formula from what we should be doing.”

According to the Canadian Network for Asthma Care, an estimated three million Canadians are living with the respiratory disease, giving the country one of the highest asthma rates in the world.

Tim Takaro, a physician-scientist at SFU, affirmed that allergy and asthma rates have increased in Canada and throughout North America and western Europe. However, he was reluctant to attribute this phenomenon solely to causes suggested by the hygiene hypothesis.

“I think it is more complicated than that,” he told the Straight. Takaro argued that a lot of uncertainty remains around the hypothesis. “Some studies show that having a lot of children in the family is protective; some show it is not,” he said.

Takaro is involved in a Canada-wide project that could shine new light on theories related to excessive cleanliness. Launching the Canadian Healthy Infant Longitudinal Development study, Takaro and scientists in Vancouver, Edmonton, Winnipeg, and Toronto have begun recruiting what will eventually be a group of 5,000 pregnant women. The study will follow these volunteers’ offspring and explore gene-environment interactions in the development of asthma and allergies.

“Hopefully a birth cohort like this one will be able to answer this question,” Takaro said.

Amin Kanani, an assistant professor in UBC’s allergy-and-immunology division, described the hygiene hypothesis as part of the reason why more people in the most-developed world are getting sick. But, he cautioned, “there are many reasons there has been a rise in allergies.”

In a telephone interview with the Straight, Kanani pointed to competing theories and noted that studies do exist that seem to contradict the hygiene hypothesis. The potential to overuse antibiotics is a concern, he said. Another is a decrease in the average person’s exposure to the sun, and therefore in the production of vitamin D.

Kanani also noted that most studies lending weight to the hygiene hypothesis focus on large population samples rather than on individual behaviour. “I don’t think that being less hygienic is going to reduce your risk of allergy,” he said.

The biggest differences in hygiene between the world’s most-developed and least-developed nations are not related to personal practices like washing one’s hands, Kanani explained. They are related to macro-level infrastructure such as sanitation services and the distribution of clean drinking water.

“In developing countries, they are not getting high rates of allergies but they’re still suffering a lot from infectious disease,” Kanani said. “I still think, on an individual level, people should be hygienic and wash their hands and be careful to not spread germs.”

Although infrastructure probably accounts for major differences in cleanliness, hygienic practices in North America are changing. A January 2007 story by the Associated Press states that between 2003 and 2007, hand-sanitizer sales in the U.S. saw growth in the double digits. More recently, a September report at Bloomberg.com found that concerns about H1N1 have translated into a 25-percent increase in share prices for Steris Corp, the largest manufacturer of sterilization products in the U.S.

With so much conflicting information out there, what’s a concerned parent to do?

Ruebush said she practises what she preaches and never gets sick. “I simply think soap and water and washing your hands is all you need,” she maintained. “Let your body do its work.”

You can follow Travis Lupick on Twitter at twitter.com/tlupick.