No one really understands the function of the sinus but occasionally the sinuses can be the site of infections. Adults and children alike experience at least two or three viral infections a year. Often viral colds or upper respiratory infections are followed by a common cold and will usually clear up on their own. After a week of having cold symptoms a viral infection can turn into a bacterial infection. When the mucus from your nasal discharge changes from clear to yellow or green , it usually indicates a bacterial infection has developed.
Sinus infections cause swelling of the lining of the sinuses and thickening of the mucus.
The swelling of the sinus tissue and thickening of the mucous causes congestion. The infection slows down the normal sinus drainage. Their are two types of Sinusitis. Acute Sinusitis, and Chronic Sinusitis. I have a Deviated Septum and that is why I'm always congested .