Works for me!
It wouldn't be too hard for her to find someone in Myrefall, since the disease pretty common.
Little bit of background information: I imagine the disease Crow has to be a waterborne illness that the villagers get from drinking the water there. Children usually don't get it because they have a bit of an immunity to it from drinking the water at a young age. The people who get it are almost always around the age of 30 or a little older, and they contract it if they get sick with a cold or something else that compromises their immune systems. Crow only got it as early as he did because he left Myrefall when he was young and lost his tolerance. Also, the other thieves didn't get sick just because they got lucky. You can think of it like if you drink tap water in Mexico. Non-natives can get sick from it, but they don't always. It's hit-or-miss depending on if you drink the specific bacterial strain that makes you ill

My knowledge of diseases and stuff is pretty limited, but I think it's fairly realistic from the little I do know xD