According to a study by the
Mayo Clinic, a sinus infection, in most cases, is caused by infected snot in your sinuses, and the toxins from the bacteria or virus or mold, and the white blood cells attacking the microbes, irritate and damage the mucus membrane. The antibiotics sometimes have a hard time getting to this infected mucus, and the mass can sometimes remain in your sinuses indefinitely, although in most people it does not.