DefinitionSinusitis refers to inflammation of the sinuses that occurs with a viral, bacterial, or fungal infection.See also: Chronic sinusitisAlternative NamesAcute sinusitis; Sinus infection; Sinusitis - acute; Sinusitis - chronic; RhinosinusitisCauses, incidence, and risk factorsThe sinuses are air-filled spaces in the skull (behind the forehead, nasal bones, cheeks, and eyes) that are lined with mucus membranes. Healthy sinuses contain no bacteria or other germs. Usually, mucus is able to drain out and air is able to circulate.When the sinus openings become blocked or too much mucus builds up, bacteria and other germs can grow more easily.Sinusitis can occur from one of these conditions:Small hairs (cilia) in the sinuses, which help move mucus out, do not work properly due to some medic