One thought that comes to mind is the theory that a certain virus or set of virus's causes schizophrenia, but only when one contracts those viruses at a very yo

One thought that comes to mind is the theory that a certain virus or set of virus's causes schizophrenia, but only when one contracts those viruses at a very young age or when one's mother has the virus during pregnancy (see http://pn.psychiatryonline.org/content/39/10/1.2.full http://pn.psychiatryonline.org/content/39/10/1.2.full). It is well-known that lower vitamin D levels lead to a weaker immune system generally, so the correlation between getting schizophrenia later in life and early-infant Vitamin D levels could suggest that the causation is actually the virus, and that the lower Vitamin D level simply makes one more susceptible to said virus.