Post-cryogenic disorder

The unnamed illness (further referred to as the post-cryogenic disorder) Nora suffered from in Fallout 4 Novelization is a side effect of prolonged cryogenic stasis. Symptoms include lower body temperature, slower metabolism, lower cold resistance, and prolonged lifespan.

Genesis
The direct reason of the post-cryogenic disorder is unspecified due to lack of other people affected by it to compare to. It was hinted at as caused by the simple length of Vault 111's cryogenic experiment, but it's also possible to have originated because of Nora being unfrozen and refrozen again - which could have lead to differing side effects, the post-cryogenic disorder being one of them.