There is much which we know of the causation of pneumonic fever; but there is vastly more of which we are, as yet ignorant. With the lapse of time and the progress of knowledge we may confidently expect that the veil will be lifted and the Egyptian darkness dispelled. In no corner of the etiological field is there so much room for doubt and error as in the study of the predisposing1 causes of diseases, and this is especially true of pneumonic fever.
This malady has been so often observed to occur during, or to follow upon some peculiar state of the general system, an injury or a disease, with such regularity and frequency as to lead to the inference that they exercise a predisposing influence over the production of the pneumonic fever. The explanation of the fact, however, is often, nay generally, not at hand. Yet even under these
WELLS EF. AN INTRODUCTION TO THE STUDY OF PNEUMONIC FEVER.CAUSATION: PREDISPOSING INFLUENCES.. JAMA. 1889;XIII(2):48-53. doi:10.1001/jama.1889.04440010012004