Recently I went to a talk by a scientist who is investigating infection (or rather immunological response to infection) as a causation for both schizophrenia and autism. There were like 4 or 5 independent factors that pointed to a relationship with infection in the second trimester. One thing I recall is the description of how an adult person with schizophrenia could in certain circumstances have their delusional symptoms disappear entirely while their body is fighting a particular infection, only to have the symptoms return when the person recovers from the illness.
"Substantial research suggests that exposure to certain illnesses (e.g., influenza) in the mother of the neonate (especially at the end of the second trimester) causes defects in neural development which may emerge as a predisposition to schizophrenia around the time of puberty, as the brain grows and develops" (Wikipedia)
Brown, AS (2006 Apr). "Prenatal Infection as a Risk Factor for Schizophrenia". Schizophrenia bulletin32 (2): 200–2. doi:10.1093/schbul/sbj052. PMC 2632220. PMID 16469941.