Don’t just let your characters come down with a nervous fever. That’s boring and hence not very dramatic.
Let them fall off a horse and break something (neck, if you want to kill them; not neck, if you want them to survive).
Let them fall off a horse and break something AND hit their poor heads on a stone so they’re out cold.
Let them fall off a horse and break something and hit their poor heads on a stone so they’re out cold AND leave them to lie there the whole night long. In a ditch. In a muddy ditch.
And after they’ve been rescued then they can come down with a nervous fever due to some psychological trauma that hit them before they dashed off into the night on their horse (from which they then fell off, etc., etc.) (you could also have the horse step on some part of its rider after it has thrown him off, just saying) (this might leave the lovely imprint of a horseshoe on interesting body parts) (just sayin’) (oh! oh! I’ve just thought of something: what if a highway robber were to come along while your poor character was lying all alone, bleeding & with a broken … uhm … leg in yonder ditch?).