This excellent review study looks at the known links between microbes and mental illness. It indicates how persistent microbial infections have been linked to numerous psychiatric illnesses and conditions, including autism, schizophrenia, bipolar, depression and anxiety.

Interestingly enough, the idea that persistent low-level microbial infections in the body and brain might be the cause of mental illness was first proposed by English surgeon and linguist John Gilchrist back in 1833. Ref: here. And in 1845, French psychiatrist Jean-Étienne Esquirol presented his ideas that psychosis might be due to germs. Ref: here. 

Certainly an infectious theory of mental illness might explain the way that many individuals experience their mental health conditions: many people only develop a mental disorder at a specific time in their life. Before that point, they may have been fine.

What might have triggered this sudden onset of psychiatric ill health? Well, catching a microbe which then forms a pro-inflammatory chronic low-level infection in the body (as many microbes do) might explain it, as many mental health disorders are now being linked to chronic low-level neuroinflammation in the brain.