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Background Studies on the effects of viral coinfection on bacterial pneumonia are still scarce in South Korea. This study investigates the frequency and seasonal distribution of virus infection and its impact on the prognosis in patients with community-acquired pneumonia (CAP). Methods The medical records of CAP patients with definite etiology, such as viruses and bacteria, were retrospectively reviewed. Their epidemiologic and clinical characteristics, microbiologic test results, the severity of illness, and 30-day mortality were analyzed. Results Among 150 study subjects, 68 patients (45.3%) had viral infection alone, 47 (31.3%) had bacterial infection alone, and 35 (23.3%) had viral-bacterial coinfection, respectively. Among 103 patients with viral infections, Influenza A virus (44%) was the most common virus, followed by rhinovirus (19%), influenza B (13%), and adenovirus (6%). The confusion-urea-respiratory rate-blood pressure-age of 65 (CURB-65) score of the viral-bacterial coinfection was higher than that of the viral infection (median [interquartile range]: 2.0 [1.0–4.0] vs. 2.0 [0.3–3.0], P=0.029). The 30-day mortality of the viral infection alone group (2.9%) was significantly lower than that of bacterial infection alone (19.1%) and viral-bacterial coinfection (25.7%) groups (Bonferroni-corrected P