退伍軍人桿菌為社區性肺炎病原菌之一，輕可為自限性疾病(self-limiting disease)，重則 可能為肺炎重症並造成死亡。臨床症狀及影像學變化雖有其獨特性，但仍難與流感病毒肺炎 區分。而早期治療可以顯著降低死亡率。本篇提出一位系統性紅斑性狼瘡患者，初始疑似流 感病毒感染的肺炎重症，隨後確診為退伍軍人桿菌感染的社區性肺炎死亡案例。藉此案例進 一步了解退伍軍人桿菌感染之各種臨床表現及與流感病毒感染之異同。

Legionella pneumophila is one of the most important pathogens that can cause severe community-acquired pneumonia. It may present as Pontiac fever, a mild and self-limiting disease, or Legionnaire’s disease, which usually causes severe pneumonia with high mortality. Although there are several unique features of Legionnaire’s disease, it remains a challenge to differentiate Legionnaire’s disease from influenza pneumonia due to the clinical and radiological similarities or overlapping between these two diseases. However, early initiation of appropriate therapy reduces mortality. Here, we present the case of a 48-year-old female patient, who had systemic lupus erythematous and underwent long-term steroid treatment. She presented with severe community-acquired pneumonia during the influenza season. Influenza pneumonia was suspected initially according to her travel history and clinical presentation, but Legionnaire’s disease was confirmed according to urine antigen assay and sputum culture. The patient died of multiple organ failure in three weeks despite aggressive antibiotic treatment. This article reviewed the clinical features of legionellosis in both immunocompetent and immunocompromised hosts, and compared the difference in clinical presentation between legionellosis and influenza infection. (J Intern Med Taiwan 2017; 28: 304-308)