The good news is that canker sores are not contagious like some other mouth sores, such as cold sores. So you can't spread canker sores by sharing food or kissing someone. Cold sores, however, are caused by the herpes simplex virus, which can pass from person to person. If you have a sore and you're wondering if it's of the cold or canker variety, just look at where it's located. Cold sores usually appear outside the mouth, around the lips, chin, or nostrils. Canker sores, on the other hand, are always found inside the mouth.