On the present, when the world brave out the rapidly spreading pandemic of COVID-19, the silent epidemic of diabetes should not be omitted. In comparison, while COVID-19 led to about 6 million of deaths in 2021, diabetes caused 6.7 million deaths in the same year. Diabetes mellitus is serious risk factor for worse outcome in COVID-19 patients. Moreover, it seems that there is a bidirectional relationship between pre-existing diabetes pandemic and rapidly spreading COVID-19 pandemic. In this article, we sum up mechanisms by which SARS-CoV-2 infects the host cell and we discuss bidirectional relationship of diabetes and COVID-19. We focus also on clinical variables in which diabetic patients differ from nondiabetic patients and which could have promising predictive value for course and outcome of diabetic COVID-19 patient’s therapy management.