From the Super Mario Wiki
Luigi, suffering from the symptoms of Veggieitis

Veggieitis is an extremely rare and bizarre disease that Luigi was once afflicted with.

In The Super Mario Bros. Super Show! episode "Flower Power", it is revealed that Veggieitis only occurs every three-hundred years in Brooklyn, and only when dehydrated vegetable seeds are eaten in spaghetti sauce; after some is afflicted with Veggieitis, they begin to spontaneously grow vegetables from their body, which, once removed, grow back faster. Luigi, who was suffering from Veggieitis throughout the episode, was eventually cured of it when he was advised by Young McDonald to eat a rose, which instantly cured Veggieitis and caused all the vegetables on Luigi's body to fall off. Unfortunately, upon eating this rose, Luigi gained another disease, one lasting a week, Roseitis, which caused roses to grow from his ears.