Mucus normally collects in sinus cavities surrounding the eyes and nose and is swept out into the nasal passages. If the narrow opening into the nasal passages is blocked by swollen tissue or a physical obstruction, the mucus collects in the sinus and infectious organisms breed there. Increased pressure in the blocked sinuses causes headache, nasal congestion, postnasal drip, thick green-yellow nasal mucus, low-grade fever, bad breath or pain in the forehead, between and behind the eyes, and in the upper teeth and cheeks.
The infection can become chronic, in which case the sinus linings no longer function properly. Sinus infections should be treated by antibiotic because they tend to be bacterial and not self-curing. If not treated, infected mucus that drips into the lower respiratory tract can cause or aggravate bronchitis, coughing and asthma. In rare cases the infection can spread to the eye or the brain, endangering one's life.