Unlike cold sores, which are caused by the herpes virus, canker sores are caused by your body essentially attacking itself. Although doctors know the immune system is responsible for creating these small and exquisitely painful ulcers, they don’t understand what starts the reaction in the first place. Once you’ve got a canker sore, avoid hot, spicy or acidic foods, which can irritate it further and hurt like heck. Rinsing with warm salt water or applying an over-the-counter topical numbing agent can help ease the pain. But don’t think you need to shy from smooching: Canker sores aren’t contagious!