Pertussis, also known as whooping cough, is a highly contagious disease that is one of the leading causes of vaccine-preventable deaths. There are 30–50 million cases per year, and about 300,000 deaths per year. Most deaths occur in children under one year of age. Ninety percent of all cases occur in third world countries. After a 7 to 10 day incubation period, pertussis in infants and young children is characterized initially by mild respiratory infection symptoms such as cough, sneezing, and runny nose. After one to two weeks, the cough changes character, with paroxysms of coughing followed by an inspiratory "whooping" sound. Coughing fits may be followed by vomiting not necessarily due to nausea but due to the sheer violence of the fit itself, which in severe cases leads to malnutrition. The fits that do occur on their own can also be triggered by yawning, stretching, laughing, or yelling. Coughing fits gradually diminish over one to two months. Other complications of the disease include pneumonia, encephalitis, pulmonary hypertension, and secondary bacterial superinfection. Pertussis is spread by contact with airborne discharges from the mucous membranes of infected people.