Type 1 diabetes develops because the body's defense system (immune system) tells the body to destroy the beta cells in the pancreas that make insulin. It's not known what causes this immune response. Some people inherit the risk for type 1 diabetes. However, even these people may not develop type 1 diabetes unless other factors, such as exposure to certain viral infections, are present.

• In infants and children, the beta cells usually are destroyed rapidly, resulting in a sudden rise in blood sugar level and a sudden onset of diabetes symptoms.
• In adults, the cells often are destroyed more slowly, resulting in a slower rise in blood sugar level and a slower onset of diabetes symptoms.

Certain genes make a person more susceptible while other genes seem to protect the person from the disease. A complex interaction between a person's genes and some environmental factors, such as having enteroviral infections-especially Coxsackie B infections-may increase the risk of developing the disease, but it does not mean that the person will definitely develop the disease.

In the past, experts thought that vaccines caused diabetes. But studies have found no link between getting a vaccination and getting type 1 diabetes.²