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Oral thrush, a fungal infection, is not considered contagious. The causative fungus, Candida albicans, is often already a natural inhabitant of the mouth and throat.
When the oral environment changes, usually due to an immature or depressed immune system, the fungus can multiply and cause symptoms.
Occasionally, individuals taking antibiotics or steroids, such as prednisone, can develop thrush.
Although thrush is usually considered not to be contagious, one instance where the fungus can be passed back and forth is between infant and mother. An infant may develop thrush and transfer it to the mother's nipples, which become painful with reddish, cracking skin. Conversely, mothers may develop a Candida albicans yeast infection on their nipples while taking antibiotics or steroids and transfer the infection to their babies while breastfeeding. The incubation period for oral thrush is about two to five days.