Okay, now onto the brain. The human body makes a lot of effort to protect the brain from invading pathogens. One of those is something called the blood-brain barrier, which is a semipermeable membrane that allows only select fluids and nutrients to pass through. But, on rare occasion, bacteria can enter as well. Because the sinuses are so close to the brain, on rare occasions an infection there can creep its way through the barrier. The most common of these rare complications is something called subdural empyema, in which pus (a big viscous mass of white blood cells) collects in the space between the outermost membrane of the brain (the dura mater) and the fine middle membrane known as the arachnoid. Once there, the infection can lead to swelling of the brain, seizures, altered mental state, and vomiting.