Other viruses have been posited as contributors to mental illness, and there's a relationship between schizophrenia and season-of-birth that may indicate prenatal/youthful infection with some otherwise mild seasonal illness (like flu) might increase the risk of schizophrenia later.
There's a strong case that Toxoplasma gondii, a communicable parasite, can contribute to schizophrenia or other mental illnesses:
http://www.cbsnews.com/news/cat-parasite-toxoplasma-gondii-l...
Other viruses have been posited as contributors to mental illness, and there's a relationship between schizophrenia and season-of-birth that may indicate prenatal/youthful infection with some otherwise mild seasonal illness (like flu) might increase the risk of schizophrenia later.