It's an auto-immune disease with complex causality. It doesn't necessarily manifest at birth, the time to develop the auto-immune response and for it to become severe varies. For some individuals perhaps it never develops.
The same propensity to develop a harmful (until my mother's generation invariably fatal) immune response attacking the body's own insulin producing cells might be an advantage to fighting off certain infections (possibly in a different genetic context or living with a different set of environmental exposures).
The same propensity to develop a harmful (until my mother's generation invariably fatal) immune response attacking the body's own insulin producing cells might be an advantage to fighting off certain infections (possibly in a different genetic context or living with a different set of environmental exposures).