A mucus membrane was a moist lining that protected various parts of one's anatomy, including the respiratory system. Repeated exposure to foreign substances could cause harm to these sensitive membranes. In 2367, Doctor Beverly Crusher said that a Tarchannen III parasite in Susanna Leijten's thymus was small enough to have entered through any of her mucus membranes. (TNG: "Identity Crisis" ) In 2371, Doctor Julian Bashir noted vascular damage to Grand Nagus Zek's mucus membranes, attributing it to the Ferengi's use of Hupyrian beetle snuff. (DS9: "Prophet Motive")