Goodpasture and Burnett[71] say: “The difficulties of analyzing the pulmonary lesions in any group of influenza pneumonias as they have appeared in this epidemic, are very apparent to anyone who has had an opportunity to observe the bacteriology and pathology of this accompaniment of the disease.” There is an acute outflow of the fluid elements of the blood and of hemorrhage into the lung tissue filling the alveoli in lobular areas and not infrequently in an entire lobe. By a special method of staining these authors have studied the distribution of Gram-negative bacilli with the morphology of B. influenzæ. The fact that in certain very early cases demonstrable bacteria of any kind are scarce or not found at all, has lead them to believe “notwithstanding the demonstration of influenza bacilli in pure culture in the lung in all but one instance, that at this stage organisms are comparatively few within the alveoli, and the primary injury is due to a very potent toxic agent elaborated in and disseminated through the larger air passages. In the later stages or from the beginning, if the injury be slight, the infection focalizes about the bronchi or their terminations, so that the bronchial and lobular distribution becomes very conspicuous.” Typical lobar pneumonia with “croupous” exudate within the alveoli occurs in cases complicated by secondary pneumococcus infection. Wolbach[72] has found that two types of pneumonia are characteristic of influenza. In cases in which death has occurred within a few days after onset of pulmonary signs, the lung tissue is dark red and “meaty in consistency” and contains abundant blood-tinged serous fluid which drips from the cut surface. The other type of lesion is found in patients who have lived for ten days or more after onset of the disease; there is extensive bronchitis, bronchopneumonia, discrete or confluent, and peribronchitis. The lungs are voluminous and the smaller bronchi are distended. Microscopically, there is peribronchitis with extensive infiltration of the interlobular septa and organization in alveoli and bronchioles. This lesion is that designated by MacCallum as “interstitial bronchopneumonia.” Wolbach thinks that the two types of lesion represent different stages of the same process. He regards as distinctive of the pneumonias of influenza the presence of hyalin fibrin lining distended air spaces. With the two types of lesion which have been described, B. influenzæ was the only organism which could be cultivated, and the author associates these distinctive conditions with that microorganism.