A sneeze is a sudden, involuntary expulsion of air from the lungs through the nose and mouth. It is often triggered by irritation in the nasal passages, which can be caused by allergens, dust, or a cold. Sneezing helps clear the nasal passages and can also spread germs if a person is sick. The act of sneezing is controlled by the brain, which sends signals to the muscles involved in the process. Sneezes can vary in intensity and sound, and they often occur in quick succession. Covering the mouth and nose while sneezing is important to prevent the spread of viruses and bacteria.