Healthy sinuses contain no bacteria or other organisms and are open, allowing mucus to drain and air to circulate. The common cold and other viral infections do not cause a sinus infection, but do inflame the upper respiratory tract (nose, sinuses, and throat), and that inflammation can increase the chances of developing a sinus infection because mucus and air cannot drain and circulate as usual. Since mucus cannot drain properly, bacteria normally living harmlessly in the nose and throat can multiply and invade the sinuses, causing an infection
The cause of sinus infection most often starts with a virus, such as the common cold, but can also be fungal. Fungi are abundant in the upper respiratory tract and are usually harmless to healthy people because the human body has a natural resistance to fungi, but can cause serious illness in people whose immune systems are not functioning properly.
There are also noninfectious triggers of sinus infection, including:
Allergies - inflammation that occurs with allergies may block your sinuses.
Deviated nasal septum - crooked septum (the wall between the nostrils) may block sinus passages.
Nasal polyps - these tissue growths may restrict the nasal passages.
Other medical conditions - the complications of cystic fibrosis, gastroesophageal reflux, or HIV and other immunodeficiency diseases may result in nasal blockage.