How to Cheat a Dragon's Curse

How to Cheat a Dragon's Curse is the fourth book in the series and revolves around Hiccup and Camicazi, Heir of the Bog- Burglars viking tribe, trying to find a cure for Fishlegs, who has Vorpentitis. Fishlegs is discovered to have Vorpentitis, and the only cure is a potato, which hasn't actually been discovered yet (except by the Hysterics tribe). Hiccup manages to steal it, only for it to be eaten by a Doomfang who also had Vorpentitis. When they get back to Berk, it is discovered that Fishlegs only had a common cold. It turns out that Hiccup has Vorpentitis. He gets Fishlegs to shoot him with an arrow that has been in the potato for the past few years (with difficulty as Stoick was trying to block him). Fishlegs shoots him in the foot and Hiccup recovers. Hiccup then plants the arrow in the ground and in a few years, it begins to sprout leaves. In a few more years, a potato begins to grow from the arrow.