The challenge

The compromised host is an individual who has become highly predisposed to (severe) infections. As a contribution to the total medical care-seeking population, this component has escalated over the last 2–3 decades. The revolution of high technological medical care (causing breaches in mechanical host defence), changes in certain socio-behavioural phenomena (leading to immune suppressive HIV disease, for example), the increasing numbers of patients with previously classified terminal disease who now are amenable to definitive therapy and who survive longer – as a result of improved techniques and therapies (for example, organ and blood/bone transplantation), the generally increased longevity of the population which provides the backdrop of increased age-related general immune disintegration and persons with more specific immune disturbing diseases such as diabetes mellitus have all contributed to produce a population much more vulnerable to infections. The impact on pulmonary disease was recently exemplified in a study of community-acquired pneumonia. In that study 57% of patients had concurrent immunosuppression – such a high figure had not been reported in previous studies. In addition to these factors, widespread appropriate and inappropriate use of antibiotics and antifungals have been associated with, and in some circumstances have selected out, difficult to treat resistant organisms, or others which would not normally have been pathogenic.

The lung is frequently involved as a target organ either primarily or secondarily to haematogenous or contiguous spread of infection in these patients.