Another possible risk factor underlying risk factor proxies is infection. For example, a possible cause of the late winter/spring excess of schizophrenia births is maternal exposure to winter born viruses.
Many groups in many countries have examined prenatal exposure to influenza as a candidate risk factor for schizophrenia with many, but not all, finding that fetuses exposed during the second trimester to the 1957 A2 influenza pandemic had an increased risk of schizophrenia. Furthermore, this link also held when the relationship between influenza epidemics and schizophrenic births was assessed over several decades. Several of the studies found an effect for females but not for males, and the findings appeared to be specific to the second trimester.
Again some but not all studies have found schizophrenia births to be increased following epidemics of other prenatal infectious agents - including diphtheria, measles, varicella and polio. In a rare study with individual data on maternal antibody levels, an association between rubella and nonaffective psychosis (including schizophrenia) has been found.
In studies with differing designs, maternal exposure to toxoplasmosis has been found to be associated with schizophrenia in offspring. Eg in a recent case-control design of a large birth cohort born, the adjusted odds ratio of schizophrenia/schizophrenia spectrum disorders for subjects with high maternal Toxoplasma IgG antibody titers was 2.61.