For people with lowered immunity, such as from HIV or cancer, however, thrush can be more serious. Untreated oral thrush can lead to more-serious systemic candida infections.
Oral thrush can also lead to a condition called burning mouth syndrome, which can have symptoms of a painful burning sensation, mouth dryness, soreness, tingling or numbness throughout the mouth and tongue, and sometimes an abnormal bitter or metallic taste in the mouth.
In severe cases of thrush, the esophagus, which leads down to the stomach, may also be involved. This will cause pain with swallowing. If someone has a weakened immune system (HIV/AIDS, cancer, chemotherapy, etc.), the Candida fungus can spread to other parts of the body and cause a systemic infection. If a person has signs or symptoms of thrush and develops a fever, shaking or chills, or difficulty swallowing, contact a doctor immediately.