Vorpentitis is a deadly sickness caused from the bite of the Venomous Vorpent. It first appeared in How to Cheat a Dragon's Curse, in which Hiccup contracts the disease (although it is first believed to be Fishlegs). The sickness can only be cured by eating or being injected with a potato or its juices in the latter sense.
When someone has been stung by a Venomous Vorpent it'll take a few months before he/she suffer the first stage of Vorpentitis. The sickness causes overheating which makes the victim feel like he/she is burning inside, causes a sore thoat, and near the end of the sickness makes body parts go dead, and causes the body to become extremely thin which makes it hard to breathe. Eventually if the victim isn't treated by a potato he/she dies.
Hiccup almost dies from the sickness but is cured when Fishlegs shoots an arrow that has been in the potato stolen from Norbert the Nutjob and therefore was coated in the potatoes juices into Hiccup's foot.
Individuals Who’ve Been Infected
Hiccup has been infected by the disease before. At the very end of How to Speak Dragonese, Hiccup, unknowingly got the disease due to a trap Alvin had placed inside of Hiccup’s Notebook. Unfortunately Alvin was about to remove it, but Hiccup took the Notebook before he had a chance.
In How to Cheat a Dragon’s Curse, Hiccup believed that his best friend, Fishlegs was infected by Vorpentitis. Because of this, he went on a long quest with Camicazi to retrieve a potato from the Hysterics. After escaping the Hysteric Tribe with the potato, the ticking thing and a peculiar arrow stuck inside the potato, the Doomfang stole the potato from Hiccup, leaving him without the potato and grief-stricken with the fact that Fishlegs would die. When he returned to Berk, he was happy to learn that Fishlegs was actually fine and only suffering a bad flu. Unfortunately Hiccup was then immediately struck down by the disease, leaving him in a deathly-ill situation. Luckily Fishlegs was able to shoot the peculiar arrow coated with the potato juices into Hiccup’s toe, thus saving him.
During How to Betray a Dragon's Hero, the Witch accidentally ate one of her fingernails, which were coated in Vorpentitis.