Candida refers to an overgrowth of Candida, a yeast fungus that normally occurs in the body Candidiasis can be local, such as oral thrush (a coating of the tongue) or vaginal yeast infections. It can also be systemic, meaning that it can spread through the entire length of the gastrointestinal tract, causing headache, fatigue, nausea, indigestion, and allergy symptoms Candidiasis is often also a co-factor in other illnesses, including Chronic Fatigue Syndrome and AIDS.