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Esophageal thrush is a yeast infection of the esophagus. The condition is also known as esophageal candidiasis. Fungi in the family Candida cause esophageal thrush. There are about 20 species of Candida that can cause the condition, but it's usually caused by Candida albicans.
More information here: https://www.healthline.com/health/candida-esophagitis#overview1