Both diseases have genetic and environmental origins. This means an individual is more at risk of developing either problem when a close relative also has it. On the genetic side, development of one reveals the pre-existing and larger risk that the genes for the other may be present. At least two genes and gene locations are connected with each disease. One gene for each disease is near one gene for the other on the same chromosome.
Nearby genes are more likely to pass together to offspring.
However, while the genes are necessary, they are not sufficient to produce the diseases. On the environmental side, researchers know gluten is needed to produce Celiac Disease, but they also know it's not the only environmental cause. With diabetes, the environmental causes are being extensively studied for prevention and cure. Type I diabetes are or will become celiac. Where the two diseases occur in one individual, in almost all cases, the diabetes is diagnosed first.