Sinus Infection (Sinusitis) Sinusitis, often called a sinus infection, happens when the lining of the sinuses becomes inflamed or swollen. This can lead to mucus buildup, pressure, congestion, facial discomfort, and drainage. Sinus symptoms often happen after a cold, allergies, or another upper respiratory infection. Most sinus infections are caused by viruses and improve without antibiotics. However, some sinus infections can become bacterial or more severe, and medical care