Pneumonia may be caused by a large variety of agents. The etiology of many primary cases can be suspected from data In the history, from epidemiologic considerations, and from physical and X-ray findings in the lungs. In the bacterial pneumonias, the diagnosis is established by isolation of the Specific inciting agent. In some of the viral and rickettsial pneumonias, the causative agent can also be isolated, but for the most part the diagnosis in these cases is retrospective and depends on the demonstration of a rise in the specific antibody in the sera during convalescence. Pneumonia may.also be an Integral part, or an early and major manifestation of, systemic Infectious diseases due to bacteria, viruses, fungi, or parasites, and of some diseases not considered to be primarily infectious. Finally, pneumonia-particularly pneumonias of bacterial origin-may occur as complications of other serious diseases or infections.