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. Virtually all deaths occur in children under one year of age. Ninety percent of all cases occur in developing countries. It is caused by certain species of the bacterium Bordetella—usually B. pertussis, but some cases are caused by B. parapertussis.