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Genetics and environment can contribute. A poor diet can lead to intestinal permeability (leaky gut) causing food particles to enter the blood stream. This can cause your immune system to react and you can be at risk of many autoimmune diseases as a result. When your immune system reacts to what it thinks are germs it can end up attacking your own body and doing damge to the thyroid in graves disease, the pancreas in diabetes and the gut in celiac disease.