Oesophageal candidiasis. Light micrograph of a section through an oesophagus infected by Candida albicans. Candida albicans (dark specks) is a fungus normally present in the mouth, which is kept under control by bacteria that also live there. If a course of antibiotics kills the bacteria, or the patient's immune system is suppressed in any way, the fungus may multiply excessively causing sore white patches (thrush). The infection can be treated with antifungal drugs.