Pertussis, a highly contagious disease of the respiratory tract, is caused by exposure to bacteria (Bordetella pertussis) found in the mouth, nose and throat of an infected person. Pertussis is primarily spread by direct contact with discharge from the nose or throat of infected individuals. Classic - or severe pertussis - as defined by the World Health Organization, consists of at least 21 days of cough illness (with the cough coming in spasms or paroxysms), associated whoops or post-cough vomiting, and laboratory confirmation. Despite widespread vaccination, reports of pertussis continue to rise in the U.S. At particular risk are newborns and babies who have not yet been fully vaccinated against pertussis, who are more likely to have severe pertussis, and who face the possibility of serious complications and death. Over the last decade, 80% of pertussis deaths have occurred in infants under 6 months of age.