Transmission is by aspiration of B.pertussis sprayed into the air by a patient, particularly in the catarrhal and early paroxysmal stages. Transmissions by contact with contaminated articles is rare. Pertussis is endemic throughout the world. One attack does not confer natural immunity for life, but second attacks, if they occur, are usually mild and often unrecognized. The incubation period averages 7 to 14 days.
Fever is rare.
The cough becomes paroxysmal after 10 to 14 days. After a few normal breaths, another paroxysm may being. Vomiting subsequent to paroxysms or due to gagging on the tenacious mucus is characteristic. The average duration of illness is about 7wk. The catarrhal stage is often difficult to distinguish from bronchitis or influenza.