Pertussis is a respiratory disease caused by the bacteria Bordetella pertussis .
Pertussis affects people all over the world and can occur at any age.
It is most commonly found in young children but can also occur in older children and adults, where it usually causes a milder illness that is often not diagnosed. Young children can get the disease from an older brother or sister or an adult with the mild form of the disease. The bacteria that causes pertussis is found in the nasopharyngeal region of the nose in infected people. Persons in the early stage of illness are the most contagious. Antibiotics will shorten the length of time the illness can be spread.