If a tuberculosis infection does become active, it most commonly involves the lungs (in about 90% of cases).[14][17] Symptoms may include chest pain and a prolonged cough producing sputum. About 25% of people may not have any symptoms (i.e. they remain "asymptomatic").[14] Occasionally, people may cough up blood in small amounts, and in very rare cases, the infection may erode into the pulmonary artery or a Rasmussen's aneurysm, resulting in massive bleeding.[9][18] Tuberculosis may become a chronic illness and cause extensive scarring in the upper lobes of the lungs. The upper lung lobes are more frequently affected by tuberculosis than the lower ones.[9] The reason for this difference is not clear.[13] It may be due to either better air flow,[13] or poor lymph drainage within the upper lungs.[