According to the authors, the modern world has become so clean, we are deprived of the bacteria our immune systems came to rely on over long ages to keep inflammation at bay."We have known for a long time that people with depression, even those who are not sick, have higher levels of inflammation," explains Raison.
"Since ancient times benign microorganisms, some times referred to as 'old friends,' have taught the immune system how to tolerate other harmless microorganisms, and in the process, reduce inflammatory responses that have been linked to the development of most modern illnesses, from cancer to depression."