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. 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 bout of 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.
Most cases of sinusitis are caused by viral infections. However, in a few people, a bacterial infection can develop. This is because when the sinus fills with mucus and empties of oxygen, it creates an ideal setting for bacteria to grow. The bacteria are often already in the nose, but don't cause an infection because they are held in check by the body's natural defenses.
If the body's defenses (e.g., the cilia, sinus drainage, or immune system) are not working properly, the bacteria can cause an infection. In rare cases, sinusitis can be caused by a fungal infection. People who get fungal sinus infections usually have other medical problems that affect their ability to fight infection (e.g., HIV, cystic fibrosis).
Other things that inflame the nose can also cause sinusitis. For example, hay fever and other allergies increase your chances of getting sinusitis.