No one knows the cause of type 1 diabetes. Clearly, one must have a genetic risk and scientists are now identifying the genetic markers. Not everyone with a genetic risk gets type 1 diabetes, however, as there also must be an environmental risk. The exact environmental insult is not clear but may be a virus and likely is different for different people. It is interesting that 90% of people with type 1 diabetes have no family history, although the genetic risk needs to be present for the disease to occur.
For type 2 diabetes, genetics appear to have a more important role as the disease usually occurs within families. Ethnic minorities (African Americans, Asian Americans, Hispanics, and Native Americans) are at a particularly high risk. Our population's trend toward a sedentary lifestyle and higher fat foods have resulted in more obesity which appears to be the main reason we are seeing more type 2 diabetes. However, not everyone who is obese gets type 2 diabetes, which speaks for the need of a genetic risk to get this disease.