Recently, it was suggested that maternal T and potentially B cells transferred during pregnancy and/or the breast milk feeding and their encounter with the antigen in mesenteric lymph nodes might play a role in development of type 1 diabetes mellitus (T1DM). T. gondii infection during gestation and/or after birth may be responsible for development of both T1DM and T2DM in children, adolescents and adults because: a) maternal microchimerism in peripheral blood was demonstrated to be significantly higher in patients with T1DM compared to unaffected siblings and healthy subjects, b) transmission of T. gondii as a Trojan horse in various types of eukaryotic cells, including T and B lymphocytes, c) swallowing by the fetus of amniotic fluid containing infected leukocytes and other cells, d) elimination of T. gondii in the breast milk during lactation, e) involvement of mesenteric lymph nodes after oral infection with the parasite, and f) damage of the myenteric neurons during infection with the parasite in both the animals with streptozotocin-induced diabetes and diabetic patients. Moreover, a significantly lower occurrence of antibodies against T. gondii found in the sera of patients with T1DM compared with their first-degree family members or healthy controls may be due to their T and/or B cell exhaustion perspective caused by chronic infection with the parasite. This suggestion may be supported by the finding that latent toxoplasmosis was associated with markedly reduced lymphocyte B-cell counts responsible for production of antibodies, markedly lower serum IgG, IgM, and IgA levels, and a significant suppression of IL-2. On the other hand, patients with T2DM had increased anti-T. gondii antibodies significantly more frequently than respective controls. Impaired vascular endothelial function characteristic for the patients with diabetes mellitus may be at least in part due to the preferential T. gondii infection of endothelial cells. Vitamin D and minocycline exerted beneficial effects on development and clinical course of diabetes mellitus probably because of their immunomodulatory and antitoxoplasmatic activities. These data strongly suggest that the parasite play an important role in development of both types of diabetes mellitus.