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 really 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.