While it is not contagious in the same way as a cold or flu virus, oral thrush can be passed from person to person in some circumstances, according to WebMD. The candida yeast that causes thrush can grow excessively when the body's immune system is compromised.
Candida yeast occurs naturally in the body and is harmless when the immune system is healthy, according to WebMD. For someone with a weak immune system, kissing someone with oral thrush could lead to the development of thrush, so it is best to avoid oral contact until the thrush has resolved. Risk factors for the development of thrush include being on certain medications such as antibiotics or corticosteroids, pregnancy or breastfeeding, diabetes, tight clothing, or damage to the lining of the mouth.