A yellow bubble on the baby’s upper lip may be related to herpes simplex virus infection and mucous gland cysts. 1. Herpes simplex virus infection: when the baby’s immune system is low, herpes simplex virus infection can easily occur, which may lead to a yellow bubble on the baby’s upper lip, accompanied by a decrease in appetite, general weakness and other clinical symptoms. 2. Mucous gland cysts: when the baby’s mucous gland ducts are blocked, mucus accumulates in the ducts, which can cause the ducts to dilate to form soybean-sized cysts for yellow bubbles. Yellow mucus can flow out after rupture. Parents should take their babies to the pediatrician of the hospital as soon as possible, and cooperate with the doctor to improve the relevant examination to clarify the cause of the disease.