The pneumonic process is very frequently associated with the typhous; but its relation to the latter, and especially to the local typhous process on the mucous membrane of the ileum, is not always the same, and hence its importance varies.

In all cases of typhus, and especially when there is well-marked ileo-typhus, there is hypostasis in the lower lobes; and this not unfrequently becomes developed into pneumonia, which deposits a gelatinous, glutinous, soft product, similar to the typhous, bronchial, and intestinal secretion, and corresponding to the existing typhous dyscrasia. It is the result of an adynamic state of the system, and bears no further definite relation to the typhous process, which is seated on the intestinal mucous membrane.

A more intimate relation, however, exists when the typhous process has been originally localized in the pulmonary mucous membrane to the exclusion of other structures, especially the intestinal mucous membrane, namely, in primary pneumotyphus; and when, in consequence of its absolute intensity or its relatively imperfect localization on the intestinal mucous membrane, it also appears in the lungs, and completes the local process on the intestinal mucous membrane, as secondary pneumotyphus.

Primary Pneumotyphus is a (croupous) lobar pneumonia characterized by the livid and almost violet color of the parenchyma during the first stage, and by a dirty brownish-red or chocolate-colored, very yielding inflammatory product (hepatization), which soon breaks down when there is great disease of the blood, and extreme absence of plasticity. It seems to be always combined with bronchial typhus, and the bronchial glands exhibit the characteristic relations of this affection. It exists either without or with only a slightly marked secondary affection of the intestinal mucous membrane, and, in association with bronchial typhus, doubtless constitutes most, if not all, of those cases of typhus, - and especially exanthematous typhus, - which run their course without any local intestinal affection. Like genuine pneumonia it is usually combined with the pleurisy yielding a similar product.

Secondary Pneumotyphus in its genuine form consists of an imperfectly developed local typhous process on the intestinal mucous membrane, has the same anatomical characters, but does not, as a general rule, attain the same degree of intensity and extent, which is presented by the primary form when it meets with no obstruction to its original local development. It also enters into the same combinations, and is very frequently associated with genuine secondary laryngotyphus.

Secondary pneumotyphus occurs, however, much more frequently in a degenerate form, as a local expression of the degeneration of the collective typhous process, and, indeed, in the form of a lobular or vesicular pneumonia yielding a purulent and diffluent product, and very frequently associated with a form of laryngotyphus which has degenerated into croup; or it occurs in the form of purulent, diffluent deposits in the interstitial tissue, with inflammation of the capillaries of the lungs (purulent metastasis); or finally in the form of pulmonary gangrene.