Mouth sores, also known as oral ulcers, are a disease of the oral mucosa, which often occur when a person's immune system is weakened. When oral ulcers occur, the symptoms can include localized redness and pain in the mucous membranes, starting with redness followed by the onset of clinical pain. Oral ulcers generally heal on their own in about two weeks. There is also a condition commonly known as mouth sores, which, clinically speaking, is specifically herpetic stomatitis caused by herpes zoster virus infection. This is characterized by small white or yellowish bumps near the corners of the mouth, containing a yellowish fluid. This condition, also popularly referred to as mouth sores, is actually a disease resulting from herpes zoster virus infection of the oral mucosa.