If a three-year-old baby develops symptoms corresponding to sinusitis, the most common cause may be allergies. The child may have allergic rhinitis in the first place. Parents may think that he can usually have symptoms such as nasal congestion, runny nose, itchy nose, sneezing, etc., but it doesn’t draw much attention to them. Once the rhinitis recurs, it can cause poor drainage of the sinuses, which can lead to sinusitis. In addition, the more common sinusitis in three-year-old children is bacterial sinusitis, which may be accompanied by the child’s headache, fever, elevated blood inflammation indicators and other manifestations. In this case, bacterial sinusitis requires appropriate anti-infective treatment, while allergic sinusitis is mainly treated with anti-allergic therapy. Also, poor drainage of mucus can lead to pediatric sinusitis. Apart from this, there are some rare pathogenic infections that cause sinusitis, such as fungal sinusitis. However, this type of sinusitis is less common in children and is only seen in immunocompromised children.