It turns out those multiple organisms are good for us. Today’s shiny clean environment is being blamed for higher rates of depression in young people.

The more hygienic our environments, the more we are deprived of bacteria and other micro-organisms that live in our gut. Our immune systems used to rely on these harmless bugs to keep us healthy. Experts call these healthy bacteria “old friends.”

“Introducing ‘old friends’ might lower the depression rates we’re seeing now, especially in young people,” says Meher Shergill, a dietitian at the Centre for Mental Health and Addiction in Toronto.

She explains the theory: There hasn’t been a huge change in our genes over the last few generations to explain the higher rates of depression, so it must be our environment. Depression rates are lower in people who live in the country – who are still exposed to soil, animals, feces – than in people who live in squeaky-clean cities. Dirt helps promote anti-inflammation in our bodies. And inflammation in the cells is linked with depression.

“There is now such a huge movement towards people being scared of germs. I understand, but people need to realize there should be moderation there.” We’re overly sanitizing everything so we’re losing exposure to bacteria that helps our immune systems. We need to give our bodies a chance to be exposed to harmless, small amounts of bacteria,” says Shergill.

Rates of depression in younger people have grown and now outnumber rates of depression in older people. Shergill treats patients who have depression, other mental illnesses and trauma.