Vitamin D deficiency (esp. in children). Without vitamin D, your bones can’t absorb calcium and become brittle and may become deformed. It can be treated, though, with vitamin D.
OMG, I read What Katy Did as a child too! (Somehow, I got the abridged version…I don’t know what the unabridged has in it.)
Now that you mention it, her fall from a swing and subsequent paralysis and sudden recovery after several years does seem a little deus ex machina.
Her father was a doctor, and I don’t remember there being a lot of medical care, other than she was in a chair, the heat from summer affected her a lot, and right after the accident she was in a lot of pain. Her cousin Helen was also paralyzed, but I think hers was a degenerative disease.
Is there a paralysis condition that can reverse itself that suddenly?
Heidi always struck me as having a very Victorian view of illness. Just get a little fresh air and exercise, and you’ll be all better!
I’ve justified the story from a modern point of view like this: Clara was ill and weak when she was a child. Her parents panicked and confined her to bed. They never let her try to walk.
Heidi did a primitave form of physical therapy. She helped Clara work her musles until she was strong enough to walk.
You’ll find the same sort of thing in “The Secret Garden.” Except in that book it specifically says there’s nothing really wrong with Colin. He was just so afraid of being a “hunchback” and dying, he refused to ever get out of bed.