Although the specific cause of diabetes mellitus is still inconclusive, it is generally accepted that diabetes mellitus is not an infectious disease. Diabetes is a large group of diseases characterized by elevated blood glucose. The etiology and pathogenesis of diabetes are extremely complex and have not yet been fully elucidated. In general, genetic and environmental factors, as well as individual lifestyle habits and autoimmune system defects, are involved in its development. Insulin is synthesized and secreted by pancreatic β-cells and reaches the target cells of various tissues in the body through blood circulation, binds to specific receptors and triggers the metabolic effects of intracellular substances. The causes are not the same for different types. Type 1 diabetes: The main causes are genetic factors or autoimmune defects that lead to abnormal pancreatic β-cell function or apoptosis and insufficient insulin secretion. In addition, environmental factors such as viral infection and stimulation by harmful substances can also cause the destruction of beta cells. Type 2 diabetes mellitus: Defective pancreatic β-cell function caused by insulin resistance and acquired factors, which are both congenital and acquired, such as obesity and metabolic syndrome. 3 Gestational diabetes mellitus: this category is generally due to changes in basal metabolism and hormone levels during pregnancy, resulting in disorders of glucose metabolism, which may either first appear during pregnancy or be triggered by previous high-risk factors. It may be related to infection of glucose metabolism by chorionic gonadotropin (HCG) and estrogen. 4 Other types: With the advancement of medical technology, some special types of diabetes mellitus with both type 1 and type 2 characteristics have been found clinically, such as autoimmune diabetes mellitus, as well as some diabetes mellitus caused by special etiologies such as abnormal insulin-producing secretion, abnormal islet alpha cell function, and abnormal intestinal flora, but the specific pathogenesis is all under study and inconclusive. In summary, although the specific etiology of diabetes mellitus is still inconclusive, the main etiologies of the different types of diabetes mellitus that have been recognized are generally unrelated to infection, so diabetes mellitus is not contagious.