Novel about two princesses in a fantasy world, one falls sick and the other travels to find a cure

Question

I'm looking for a story that I read back when I was a child. Around 2005, maybe? I recall the book seeming pretty old, but it was from a public library, so it could be simply deteriorated by usage.

The world is being afflicted with a mysterious sickness that has no cure and no one knows where did it come from (it is called gray fever, gray plague or something like that).

The story narrates two young princesses, the elder one would be brave and adventurous, while the younger one would be shy and more delicate. They both fantasized with a future where the younger one would fall sick and the elder one would depart trying to find a cure to save her.

But in reality, the elder one falls sick and the young one is forced to depart. I recall some sort of mage/warlock that helps her giving her a pair of boots that would let her walk big distances almost instantaneously. I also recall the younger one being trapped by a dragon, and in the end the elder one dies from the sickness, but is saved being transformed in a fairy. I also think the sickness is eradicated in the end.

Answer 1

A Google search on "sick princess transformed fairy" quickly reveals the answer: The Two Princesses of Bamarre, a 2001 novel by Gail Carson Levine, the author of Ella Enchanted.

From Wikipedia:

The story revolves around the lives of two sisters who are very close, but as different as night and day. When one of them falls victim to a deadly disease sweeping the kingdom, the other must find her inner strength and bravery to discover the cure, and save her elder sister.

(...)

Meryl also tells Addie that she, too, was one of those on the brink of death when the rain came, so the fairies could not truly save her. However, they offered to transform her into a fairy and join them in an endless battle against fearsome, monstrous creatures, the outcome of which affects the world below. Meryl accepted the offer, and is now a fairy, unable to return with Addie.