Acute sinusitis is usually precipitated by an earlier upper respiratory tract infection, generally of viral origin. If the infection is of bacterial origin, the most common three causative agents are Streptococcus pneumoniae, Haemophilus influenzae, and Moraxella catarrhalis.^[4] Until recently, Haemophilus influenzae was the most common bacterial agent to cause sinus infections. However, introduction of the H. influenza type B (Hib) vaccine has dramatically decreased H. influenza type B infections and now non-typable H. influenza (NTHI) are predominantly seen in clinics. Other sinusitis-causing bacterial pathogens include Staphylococcus aureus and other streptococci species, anaerobic bacteria and, less commonly, gram negative bacteria. Viral sinusitis typically lasts for 7 to 10 days,^[4] whereas bacterial sinusitis is more persistent. Approximately 0.5% to 2% of viral sinusitis results in subsequent bacterial sinusitis. It is thought that nasal irritation from nose blowing leads to the secondary bacterial infection.^[5]

Acute episodes of sinusitis can also result from fungal invasion. These infections are typically seen in patients with diabetes or other immune deficiencies (such as AIDS or transplant patients on immunosuppressive anti-rejection medications) and can be life threatening. In type I diabetics, ketoacidosis can be associated with sinusitis due to mucormycosis.^[6]

Chemical irritation can also trigger sinusitis, commonly from cigarette smoke and chlorine fumes.^[7] Rarely, it may be caused by a tooth infection.^[4]