Boils in the mouth, or mouth sores, are caused by infections such as mononucleosis, herpes, syphilis and human immunodeficiency syndrome, as detailed by Healthline. Additional causes of mouth sores include cancer, anemia, Crohn's disease, lupus and allergies to certain medications.
There are various types of mouth sores that can appear on the gums, cheeks, tongue or lips, according to MedlinePlus. Mouth sores are not always caused by infection, and they can result from irritation that occurs due to braces, biting an area of the mouth, burning the mouth and poorly fitted dentures. Two common types of mouth sores are cold sores and canker sores. Cold sores are caused by the herpes simplex virus and are extremely contagious, while canker sores are not contagious. Typically, cold sores look like blisters and develop a crust over them, and canker sores look yellow or pale in color, resembling an ulcer with a red ring around the outside.
The reason canker sores occur is unclear, but they may develop due to a weakened immune system as a result of an illness such as the flu, hormone changes, stress and certain vitamin deficiencies, as MedlinePlus explains. Cold sores caused by the herpes simplex virus only appear as mouth sores when something else triggers the virus, such as sun exposure, another illness, hormone changes or stress. It is less common for mouth sores to develop due to more serious conditions, such as tumors, illnesses and reactions to certain medications.