Acute sinusitis is usually caused by a viral infection, bacteria or an allergy. The infection starts after something blocks the opening to the sinuses, most commonly caused by a viral infection such as a cold.
During a cold, the mucous membranes (lining) of the nasal passage tend to block the opening of the sinuses. The air that usually fills the sinuses is absorbed into the bloodstream, and the pressure in the sinuses reduces, drawing fluid into the sinuses and causing pain. This fluid then may become a breeding ground for bacteria.
Your white cells (immune cells) and more fluid enter into the sinuses to fight the bacteria. This increase in fluid then causes the pressure to increase in the sinuses resulting in more pain. The infection, however, will often remain viral before clearing. It can less frequently become bacterial, where bacteria add on to the infection caused initially by a virus. This tends to cause the symptoms to become worse and to last longer.
Infrequently the infection may spread from a dental abscess to the sinus in the cheekbone (maxillary sinus).