The Sleeps by Colbby

After an accident in space Jay crash-lands onto a strange world and loses his ship. He is rescued, but enslaved. After many adventures he manages to win his freedom and together with his new wife starts life in his adopted land. While visiting relatives of his wife, their son is infected with a fever from a swamp and lapses into a coma. Jay takes him to see the Regal of Mordia whose surgeons diagnose the fever as the Sleeps. The only cure is a medicine made from a tree root, but it comes from a land far away where people disappear and are not heard of again.