It's the nature of this sickness that makes the story sf - one fine morning, he wakes up a giant "vermin"! No explanation is offered for this, but that's not important either - story is really about how the family tries to cope with it.
The story is not always logical & is occasionally too verbose, but it's sometimes very moving.
Fact sheet.First published: 1915 (as a separate book in German?)
Download full text of an English translation by David Wyllie from Project Gutenberg. [via Best Science Fiction Stories]
PS: This post is actually based on a different translation that's not online.
Image above courtesy is of The Rhyming Med Student.