One of the most common causes of pus in the ears of a three month old baby is acute otitis media. In children, the Eustachian tube is short, wide and has a low, flat opening, so if you have a cold or are not breastfeeding correctly, bacteria can enter the middle ear through the Eustachian tube and cause acute otitis media. Acute otitis media can be characterized by pain in the ear, pus and water, and because children are unable to express themselves, they may cry and scratch their ears. The systemic symptoms of otitis media in children are more serious than those of adults, and may include high fever, diarrhea, vomiting and other systemic symptoms. Therefore, it is recommended that three-month-old babies with pus in the ear be taken to the hospital for examination and treatment in a timely manner, so as to avoid delays in the condition, which may lead to complications or the development of chronic otitis media that may affect the child’s hearing.