An autoimmune reaction (when the body attacks itself by mistake) causes type 1 diabetes. Beta cells, which make insulin in the pancreas, are destroyed in this reaction. It may take months or years for symptoms to appear after this process has begun.   People with certain genes (traits passed from parent to child) are more likely to develop type 1 diabetes. Despite having the genes, many of them will not develop type 1 diabetes. A trigger in the environment, such as a virus, may also play a role in type 1 diabetes development. Type 1 diabetes, according to medical professionals, is not caused by diet or lifestyle habits.