Sinusitis usually results from a bacterial infection (Streptococcus pneumoniae, Haemophilus influenzae, anaerobes) or, less frequently, from a viral infection. Viral sinusitis usually follows an upper respiratory infection in which the virus penetrates the normal mucous membrane, decreasing ciliary transport.

Fungal sinusitis is uncommon and found more often in immunocompromised patients. The most common types are aspergillosis, mucormycosis, candidiasis, histoplasmosis, and coccidioidomycosis. The spores causing these infections are usually found in soil and enter through the respiratory tract.

Acute sinusitis is caused most commonly by H. influenzae, Staphylococcus aureus, S. pneumoniae, and Streptococcus pyogenes. Predisposing factors include any condition that interferes with sinus drainage and ventilation, such as chronic nasal edema, a deviated septum, and viscous mucus. Bacterial invasion also may result from swimming in contaminated water. Generalized debilitating conditions, including chemotherapy, malnutrition, diabetes, blood dyscrasias, long-term steroids, and immunodeficiency, may also predispose an individual to sinusitis.

Complications—typically resulting from inadequate therapy during the acute phase or from a delay in treatment—may include meningitis, cavernous sinus thrombosis syndrome, bacteremia or septicemia, brain abscess, frontal lobe abscess, osteomyelitis, mucocele, and orbital cellulitis or abscess.