Oral thrush is characterized by ulcers or lesions in the mouth caused by the yeast fungus Candida albicans. The lesions are painful, slightly raised, whitish in appearance, and cause a dry mouth. Update Date: 01/18/2007Benjamin W. Van Voorhees, MD, MPH, Assistant Professor of Medicine and Pediatrics, The University of Chicago, Chicago, IL. Review provided by VeriMed Healthcare Network.