John M. Eagles suggested that polioviruses might cause schizophrenia because 1) several reports of a recent decline in the incidence of schizophrenia coinciding with the introduction of polio vaccination, 2) the observed winter excesses in schizophrenic births (in temperate climates) could be explained by fetal exposure to poliovirus during the second trimester of gestation which would occur during the summer when polio epidemics are most frequent, 3) there are increased rates of schizophrenia among immigrants to the UK from regions of the world with low frequencies if immunity to polioviruses, 4) there may be genetic variants in the poliovirus receptor gene that could increase susceptibility to poliovirus infection (1). The large discordance rates for schizophrenia in monozygotic twin pairs indicate the existence of both genetic and environmental factors. Numerous genetic studies indicate an interaction of several genes in the etiology of schizophrenia. These genes may encode a family of poliovirus receptor subunits, various active combinations of which are expressed on T-immunocytes, monocytes, endothelial cells, and limited populations of (glutamatergic?) neurons. The poliovirus receptor on the T-cell may require both a specific combination of V segments of the T-cell antigen receptor, as well as a specific major histocompatibility (MHC) antigen, acting in concert to infect monocytes, the primary transporter of poliovirus from blood into the brain. The very large discordance rates for schizophrenia that probably exist for dichorionic-monozygotic twins (about 90%), as well as the much smaller discordance rates for monochorionic-monozygotic twins (about 40%), may be due to several allelic exclusion events expressed both in T-cells and possibly in certain neurons. A child who has lost some glutamatergic neurons due to viral infection during the second trimester of gestation, may be able to compensate for this deficit to a large extent by the super-abundance of excitatory synapses that exists in the brain until sexual maturity, at which time a selective loss of excitatory (mainly glutamatergic) synapses occurs together with hormonally induced changes in behavior, leading to a much increased risk of a psychotic episode.