Etiology of community-acquired pneumonia in patients requiring hospitalization.

The etiology of community-acquired pneumonia was studied in 127 patients with roentgenologically verified pneumonia who needed hospitalization. Etiology was determined on the basis of a positive blood culture and/or a significant antibody titer increase. Streptococcus pneumoniae was the probable etiological agent in 69 patients, nontypeable Haemophilus influenzae in five patients, Streptococcus pyogenes in two patients, and Legionella pneumophila and Staphylococcus aureus in one patient each. Evidence of Mycoplasma pneumoniae infection was found in 18 patients and of Chlamydia psittaci infection in three patients. Influenza virus type A was the cause of infection in 15 patients. One patient had infection with influenza virus type B, one patient with parainfluenza virus type 1, and three patients with respiratory syncytial virus. In 20 patients there was evidence of infection with more than one microorganism. No etiological agent was found in 27 patients. Since Streptococcus pneumoniae was the predominant etiological agent penicillin should be drug of first choice in patients with pneumonia who need treatment in hospital. In young adults, however, the high frequency of Mycoplasma pneumoniae infection would justify the use of erythromycin or doxycycline as drug of first choice.