Causes
The sinuses produce mucus that helps us clean the nose and to smell.
It's moved out of the sinuses and into the nasal cavity by thousands of tiny
hairs, called cilia, which operate in unison to form a sort of conveyor
belt. Foreign particles and organisms entering the sinuses land in the mucus
and are sent back to the nose, away from danger. To get to the nose, the mucus
has to pass through small holes in the bones that surround the sinuses.
Sinusitis usually begins during a common cold, influenza (flu), or some other
viral infection. This causes the nasal mucous membrane (which is soft tissue
inside the nose, not simply mucus) to swell. It can press against the hole through
which mucus leaves a sinus. The sinus fills with mucus and empties of oxygen,
creating an ideal setting for bacteria to grow.
The bacteria are often already in the nose, but don't cause any symptoms.
Humans are typically infested by organisms that are held in check by the
body's natural defenses. If something goes wrong with the defensive system,
they can cause medical problems like sinusitis.
Apart from viral infections, anything that inflames the nose can cause sinusitis.
Hay fever, for example, increases your chances of getting sinusitis.
People with diabetes, cystic fibrosis, or AIDS are at increased risk, as are
those who have had their nose broken previously and those who were born with
a malformed septum (the dividing wall between the nostrils).