Sneeze. High-speed photograph of a jet of droplets erupting from a man's mouth as he sneezes. The sneeze reflex is involuntary and is usually triggered by irritation of the nasal lining, such as occurs during a common cold or influenza (flu). The ejection of fluid helps to cleanse the respiratory tract of dust filtered out of the air, or mucus produced in response to infection. The reflex is exploited by the cold and flu viruses, which uses sneezing as a means of transmission from person to person.