In the control group IgG was positive in 36 (36 %) subjects but IgM antibody positivity was not detected.<h4>Conclusions</h4>According to the study results, there was a statistically significant relationship between the presence of <i>Toxoplasma gondii</i> seropositivity and Parkinson's and Alzheimer's diseases, but there was no a statistically significant relationship in multiple sclerosis patients. The gene discussed is CD40LG; the disease is early-onset autosomal dominant Alzheimer disease.