Toxoplasma serostatus (international units) is measured using solid-phase enzyme immunoassay at Stanley Laboratory of Developmental Neurovirology at the Johns Hopkins School of Medicine. Researchers performing the assays are blinded to participants’ clinical characteristics. Specimens are classified as seronegative if the immune status ratio, determined by comparison with defined standards, is less than 10.0, or as seropositive if the ratio is ≥10.0.