The infective agent in this disease is quite unknown, although many attempts have been made to discover it. The indications are that it lodges in the respiratory passages, where its toxine produces irritation resulting in catarrh and the violent paroxysmal coughing which characterizes the disease. It is probable that there is not from the pure infection much general intoxication. There is frequently, beyond a simple catarrh, a serious broncho-pneumonia, which greatly raises the mortality of whooping cough. But whether this is the result of the direct infection or of a secondary infection of the ordinary pyogenic microbes remains as yet obscure.