Pertussis, also known as whooping cough, is a respiratory illness caused by the highly contagious bacterium, Bordetella pertussi. Pertussis can cause severe disease, especially in infants younger than one year. Individuals that are immunocompromised or have asthma also have a higher risk of severe illness if infected. Pertussis primarily affects infants that are too young to receive the vaccine as well as older children and adults whose protection from the vaccine has waned.