So I've been re-reading Goblet of Fire and after reading Barty Crouch Jr's recounting of how he and his mother switched places in Azkaban via Polyjuice, I'm unclear why Mrs. Crouch died while she was still taking Polyjuice.
If Mrs. Crouch's body was terminally ill, each time she took the Polyjuice wouldn't she have had Barty Crouch Jr's body, which was not terminally ill? Why, then, did she die very soon after Barty Crouch Jr was smuggled out of Azkaban? Shouldn't she have had her son's strength and health from the Polyjuice? Yes, Barty Crouch Jr was weakened by Azkaban, but he wasn't dying.
Deathly Hallows shows that the physical limitations or disabilities of the person providing the hairs for the Polyjuice Potion are transferred to the person taking the potion:
‘Harry, your eyesight really is awful,’ said Hermione, as she put on glasses.
Deathly Hallows -- page 49 -- Bloomsbury -- chapter 4, The Seven Potters
It would seem the same would go for healthy characteristics. As long as Mrs Crouch had a supply of Polyjuice Potion, shouldn't she have remained alive? It could be said that she ran out of Polyjuice and then died, but Barty Crouch Jr says implicitly that his mother died with Barty Jr's appearance, meaning the Polyjuice was still in her system and working at the time of her death.
Why did Mrs. Crouch die while taking Polyjuice Potion?