Scientific American reports:
Studies have repeatedly linked schizophrenia to prenatal infections with influenza virus and other microbes, showing that the children of mothers who suffer these infections during pregnancy are more likely to be diagnosed with schizophrenia later in life. In 2006 scientists at Columbia University asserted that up to one fifth of all schizophrenia cases are caused by prenatal infections.
Doctors have known for many years that microbes such as syphilis and Streptococcus can, if left untreated, lead to serious psychiatric problems. Now a growing number of scientists are proposing that microbes are to blame for several mental illnesses once thought to have neurological or psychological defects at their roots. The strongest evidence pertains to schizophrenia, but autism, bipolar disorder and obsessive-compulsive disorder have also been linked to bacterial, viral or parasitic infections in utero, in childhood or in maturity. Some of these infections can directly affect the brain, whereas others might trigger immune reactions that interfere with brain development or perhaps even attack our own brain cells in an autoimmune mistake.
It's not a new idea either:
In 1896 Scientific American published an editorial entitled “Is Insanity Due to a Microbe?” The question seemed logical, given that microbes were starting to be implicated in other diseases. In the editorial, two doctors described how they had injected cerebrospinal fluid of mentally ill patients into rabbits, which later got sick. The doctors concluded that “certain forms of insanity” could be caused by infectious agents, “similar to typhoid, diphtheria and others.”