A 25-year-old man presents with a progressive illness of several days' duration characterized by nonproductive cough, fever, and malaise. A lateral view chest radiograph reveals platelike atelectasis. Elevated titers of cold agglutinins are detected. Which of the following is the most likely type of pneumonia in this patient?

Interstitial (primary atypical) pneumonia is most commonly caused by M. pneumoniae or viruses. Interstitial pneumonia is characterized by diffuse, patchy inflammation localized to the interstitial areas of alveolar walls, with no exudate in alveolar spaces, and intra-alveolar hyaline membranes. M. pneumoniae infection is associated with the presence of cold agglutinins, which are IgM antibodies that react nonspecifically with all human red blood cells. P. jiroveci (carinii) pneumonia is the most common oppounistic infection in patients with acquired immunodeficiency syndrome Viral pneumonias are the most common type of pneumonia in childhood, caused most commonly by the influenza virus.