PNEUMONIA. 355 occurs. Such a pneumonia may be caused by the tubercle bacillus alone, but more often it is aided by accompany- ing staphylococci, streptococci, tetragenococci, pneumo- cocci, pneumobacilli, and other organisms apt to be pres- ent iu a lung in which tuberculosis is in progress and ulceration and cavity-formation are advanced. 4. Mixed Pneumonias.—It frequently happens that pneumonia occurs in the course of, or shortly after the convalescence from, influenza. In these cases a mixed infection is present, and there is no diiBculty in deter- mining that both the influenza bacillus and the pneumo- coccus are present. Again, sometimes the pneumococci and staphylococci operate simultaneously, and produce a purulent pneumonia with abscesses as the conspicuous feature. As almost any combination of the described bacteria is possible in the lungs, and as these combi- nations will all produce varying inflammatory conditions, it must be left for the student to imagine what the par- ticular characters of each may be. Among these mixed pneumonias may be mentioned those called by Klemperer and Levy " complicating pneumonias," occurring in the course of typhoid, etc.