Sinusitis is almost always due to an infection, although swelling from allergies can mimic the symptoms of pressure, pain, and congestion; and allergies can set the stage for a bacterial infection. Bacteria are the most common cause of sinus infection. Streptococcus pneumoniae causes about 33% of all cases, while Haemophilus influenzae causes about 25% of all cases. Sinusitis in children may be caused by Moraxella catarrhalis (20%). In people with weakened immune systems (including patients with diabetes; acquired immunodeficiency syndrome or AIDS; and patients who are taking medications which lower their immune resistance, such as cancer and transplant patients), sinusitis may be caused by fungi such as Aspergillus, Candida, or Mucorales.

Acute sinusitis usually follows some type of upper respiratory tract infection or cold. Instead of ending, the cold seems to linger on, with constant or even worsening congestion. Drainage from the nose often changes from a clear color to a thicker, yellowish-green. There may be fever. Headache and pain over the affected sinuses may occur, as well as a feeling of pressure which may worsen when the patient bends over. There may be pain in the jaw or teeth. Some children, in particular, get upset stomachs from the infected drainage going down the back of their throats, and being swallowed into their stomachs. Some patients develop a cough.

Chronic sinusitis occurs when the problem has existed for at least three months. There is rarely a fever with chronic sinusitis. Sinus pain and pressure is frequent, as is nasal congestion. Because of the nature of the swelling in the sinuses, they may not be able to drain out the nose. Drainage, therefore, drips constantly down the back of the throat, resulting in a continuously sore throat and bad breath.