Many factors, both genetic and environmental, have been blamed for increasing the risk of a diagnosis of schizophrenia. Some, such as a family history of schizophrenia, are widely accepted. Others, such as infection with Toxoplasma gondii, a parasite transmitted by soil, undercooked meat and cat feces, are still viewed with skepticism. A new study used epidemiological modeling methods to determine the proportion of schizophrenia cases that may be attributable to T. gondii infection. The work suggests that about one-fifth of cases may involve the parasite.