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Imagine coughing so hard and for so long that you turn blue and stop breathing. Pertussis, or whooping cough, can do that to an infant. The disease is caused by the bacterium Bordetella pertussis and occurs in three stages. The catarrhal stage, characterized by runny mucous, is highly contagious. It’s followed by the paroxysmal stage—unstoppable, sustained, violent coughing accompanied by a “whoop” when you inhale.