People with diabetes (type 1 and 2), even when well-managed, are at increased risk of severe disease and complications, like hospitalization and even death, as a result of getting the flu. This is because diabetes can make the immune system less able to fight infections. In addition, illness can raise your blood sugar level. Also, sometimes people don’t feel like eating when they are sick, and this can cause blood sugar levels to rise and fall.

http://www.cdc.gov/flu/diabetes/