Canker sores are caused by nutrition deficiency and stress. They are NOT contagious in any way and are not caused by any forms of STDs. The answer above describes a Cold Sore, often mistaken with...
They will go away by themselves within 4-14 days. To lessen the pain you can buy gel (e.g. bonjela) or special mouthwash (e.g. colgate peroxyl, oraldene). Gargling with extremely salty water is also...