Viral Sinusitis: This is the most common cause of sinusitis, accounting for approximately 90-98% of cases. The symptoms are often similar to those of bacterial sinusitis, making it challenging to distinguish between the two without further testing.

Bacterial Sinusitis: This is a common cause of sinusitis, especially in cases where symptoms persist or worsen over time. Bacterial sinusitis is more likely if symptoms last for more than 7-10 days or if there is a sudden worsening of symptoms.
Allergic Rhinitis: This condition can cause similar symptoms to sinusitis, such as nasal congestion and discharge, and can be a contributing factor to the development of sinusitis.
Anatomical Abnormalities: Deviated septum, nasal polyps, or other anatomical issues can increase the risk of developing sinusitis.

Invasive Fungal Sinusitis: Although rare, this condition can be life-threatening, especially in immunocompromised patients. It requires prompt diagnosis and treatment.
Cavernous Sinus Thrombosis: A rare but potentially life-threatening complication of bacterial sinusitis, characterized by the formation of a blood clot in the cavernous sinus.
Orbital Cellulitis: An infection of the tissues surrounding the eye, which can be a complication of bacterial sinusitis.

Tuberculous Sinusitis: A rare cause of sinusitis, especially in areas where tuberculosis is common.
Syphilitic Sinusitis: A rare cause of sinusitis, usually seen in patients with advanced syphilis.
Aspergillosis: A rare fungal infection that can cause sinusitis, especially in immunocompromised patients.
Wegener's Granulomatosis: A rare autoimmune disorder that can cause sinusitis, as well as other systemic symptoms.