Nasal polyps can develop in the mucous lining of your nose or in one or more of your sinuses — four hollow cavities above and behind your nose. But polyps aren’t a disease. Rather, they’re the end product of ongoing inflammation that may result from viral or bacterial infections, from allergies or from an immune system response to fungus. Chronic inflammation causes the blood vessels in the lining of your nose and sinuses to become more permeable, allowing water to accumulate in the cells. Over time, as gravity pulls on these waterlogged tissues, they may develop into polyps.