I think infectious disease > inflammation of brain > depression is well established. And not just depression, but a whole range of behavioral and psych impairments.

Thousands of years ago the Chinese considered parasitic infections to be a form of demonic possession. Why? Because they observed that sufferers often exhibited personality changes, volatile emotions, and other mental disturbances that were so severe it seemed they were possessed. Western medicine is largely clueless on this.

An herbalist I saw for a while said:
"I often find that people are 'mentally ill' because of physical causes that facilitate an instability of their brain chemistry. Inflammation of the nervous system by pathogenic viruses and spirochetes [as in Lyme disease] is especially common in these patients."