The nasal congestion that results blocks the sinuses, preventing drainage of mucus into the nose
and mouth. The sinus mucus then builds up and thickens, and infection
in the sinuses becomes more likely. Infectious sinusitis can also be caused by a bacterial infection or sometimes by a fungal infection.
Allergic sinusitis is caused by an allergic reaction to dust, mold, pollen, or another substance in the environment.