Many diseases and conditions may begin with sores (lesions) on the mucus membranes of the gums and inner cheeks.

Origins of lesions can be grouped into several broad categories:

• Infection
• Autoimmune diseases
• Mechanical irritation
• Bleeding disorders
• Malignancy

The majority of mouth lesions result from mechanical irritation (such as biting the cheek or from braces) and minor infections (hand-foot-mouth syndrome, herpangina, herpes stomatitis, cankers).

Mouth sores may be the first sign of AIDS, with lesions such as hairy leukoplakia and opportunistic infections such as oral candida.

The herpesvirus often resides in the body for years, manifesting itself as mouth sores only when another illness causes a rise in body temperature (fever), following an injury (such as accidentally biting the inside of the lip), or during periods of stress.

Although it is not known for sure, canker sores may not be caused by a virus, but may be the result of a temporary malfunction of the immune system. Women are more likely to get cankers than men, but anyone can get them for no apparent reason.

• Fever blisters and cold sores caused by a herpesvirus
• Canker sores
  • Common in children, especially those wearing dental braces
  • May be related to a virus
  • May be related to an injury to the mucosal lining of the mouth
• Coxsackievirus (usually accompanied by spots on the hands and feet, sometimes called hand-foot-mouth syndrome)
• Syphilis (transmitted by oral sex)
• Trauma
• Drugs such as chemotherapeutic agents
• Drug allergies including reactions to aspirin, barbiturates, gold, penicillin (Penicillin V), phenytoin, quinine, streptomycin, or sulfonamides