Background

“Pneumonia”, wrote Laennec, “is one of the diseases most anciently known; and before pathological anatomy . . . had investigated the true nature of diseases, it was generally regarded as one of the internal afflictions most readily recognized.”

Pneumonia occurs when the host mounts an inflammatory response, centered on the lung parenchyma, usually against a microorganism, but sometimes against another toxic agent, which has reached this normally sterile site. Bacteria are the most common causative microorganisms. The effect on the host is variable, ranging from complete absence of clinical manifestations to sudden death or a brief illness followed by sudden death. More typically the effects of the inflammatory response and the replacement of the normal gas-exchanging lung tissue cause a constellation of symptoms and signs. These are associated with, and may be diagnosed as, lung infection. Untreated, the condition progresses until the host dies or the inflammatory response overcomes the microbial threat and lysis of the toxic state is followed by gradual recovery. Recognition of the condition will usually lead to appropriate medical intervention, resulting in improved outcome in most cases.