In cancer, corrupted neoplastic cell successfully evades the immune attack using two main strategies: avoiding the immune recognition and instigating an immunosuppressive tumor microenvironment. Cancer is able to induce immune tolerance and present itself to immune system as normal tissue. There is a defect in immune tolerance: the tolerance is wrongly induced toward cancer. In autoimmune diseases, immune system attacks normal tissue because it fails to recognize its own constituent parts as self. There is a defect in immune tolerance: the tolerance is lost toward normal tissues. In both diseases wrong information in the communication between tissue (neoplastic) cells and immune system is crucial: in cancer, neoplastic cells avoid immune attack, whilst in autoimmune diseases, immune system attacks normal cells. Diabetes mellitus type I and II are associated with an increased risk of developing different types of cancer. Pathogenesis of diabetes mellitus type I and pathogenesis of cancer overlap in genetic, environmental and immunologic factors. Diabetes mellitus type II and obesity can both induce oncogenesis.