Harkening back to Adolph Meyer’s early 20th century theory, Canli notes how certain infections of the brain – perhaps most notably Toxoplasma gondii — can result in emotional disturbances that mimic psychiatric conditions. He also notes that numerous pathogens have been associated with mental illnesses, including Borna disease virus, Epstein-Barr and certain strains of herpes, including varicella zoster, the virus that causes chickenpox and shingles.

A Danish study published in JAMA Psychiatry in 2013 looked at the medical records of over three million people and found that any history of hospitalization for infection was associated with a 62 percent increased risk of later developing a mood disorder, including depression and bipolar disorder.

Canli believes that pathogens acting directly on the brain may result in psychiatric symptoms; but also that autoimmune activity — or the body’s immune system attacking itself — triggered by infection may also contribute. The Danish study also reported that a past history of an autoimmune disorder increases the risk of a future mood disorder by 45 percent.