A cold or viral upper respiratory infection causing swelling that blocks the tube connecting the nose to the middle ear.
The Eustachian tube in young children being shorter and flatter than in adults, making it easier for bacteria and viruses to reach the middle ear - this explains why ear infections are far more common in children under five.
Enlarged adenoids blocking the drainage pathway and harbouring bacteria that contribute to recurrent infections.
Glue ear - thick fluid accumulating in the middle ear without acute infection - often following repeated ear infections or persistent Eustachian tube dysfunction.
Exposure to cigarette smoke, which is a well-established risk factor for both acute and recurrent ear infections in children.
Rosenfeld RM et al. Clinical Practice Guideline: Otitis Media with Effusion. Otolaryngol Head Neck Surg. 2016;154(1 Suppl):S1-S41. NICE guideline NG91: Otitis media (acute). 2018.