The pleura in the course of influenza is affected as a result of influenza pneumonia, which may be followed by a fibrinous, serous, or purulent pleurisy. Hemorrhagic exudates are exceedingly rare. But a primary pleurisy is by no means rare. Kundrat, on the basis of pathologic investigations, calls attention to a special peculiarity of influenza bronchitis "that, without pneumonic areas or pus formation, purulent pleurisy frequently supervenes." The same condition is also mentioned by Kahler, Weichselbaum, Netter, Mayor, and others. R. Pfeiffer found, in two cases of empyema of the pleura, " enormous quantities of influenza bacilli in pure culture, and for the most part inclosed in the protoplasm of the pus corpuscles; it is therefore proved that influenza bacilli can find their way to the surface of the lung, and may there give rise to a purulent exudate"; in three other cases of empyema following influenza the specific microorganisms were not found. "They were replaced by streptococci or the diplococcus of Frankel, and hence were due to a secondary infection." The bacteriologic etiology of this form of pleurisy is therefore quite clear.

Of great importance is an extraordinarily severe, usually fatal form of acute, primary influenza pleurisy, which may begin simultaneously with the influenza attack or more frequently develops upon the second or third day. This complication begins with a rigor and high continued fever; severe dyspnea and cyanosis set in early; in a short time a rapidly developing; often bilateral, very characteristic exudate is formed. The fluid is thin, opaque, and sero purulent, of a peculiar pale yellow color; we have used the expression, " Weincreme like"; by others (Furbringer) this exudate has been appropriately named "mortar like." By several autopsies we convinced ourselves* that these acute exudative grip pleurisies were primary-that is, apart from the diffuse severe bronchitis and hyperemia of the lung there was no inflammatory infiltration of the lungs. In the exudate, as we noted in our lectures in 1890, streptococci were frequently found in pure culture.

The complication of a diffuse, especially capillary, bronchitis or bronchopneumonia with pleural exudation, even in a slight amount, produces severe dyspnea, cyanosis, and marked cardiac asthenia. Even robust young individuals may be carried off in this manner within a few days, as the cases reported by us and A. Vogl (Munich) show.

* Loc. ext., pp. 4 and 57.

Gerhardt, Curschmann, and Heubner very properly called attention to the frequency of pleurisy complicating influenza pneumonia. Auer bach remarks. "These dry pleurisies are frequently distributed over the entire surface of the lung." Among the pathologists, Zahn called attention to the "extraordinarily frequent pleural exudates in catarrhal influenza pneumonia, which in the ordinary catarrhal pneumonia is rare." The German collective report agrees well with this statement. Among the 3185 observers, not less than 869 (or 27 per cent.) called special attention to pleurisy unaccompanied by pneumonia as a sequela of influenza (Fr. Strieker).