人體體內共生著由許多種類的微生物所形成的菌落，這些細菌可以在皮膚與消化道等全身各器官系統存在。這些為數眾多的細菌、古細菌、真菌和病毒統稱為“微生物群”(microbiota)。部分的微生物群可以透過各種直接與間接機制影響多種生理病理功能，造成宿主的健康與疾病狀態，反之，如果宿主有病態生理狀況下，也會造成微生物菌群組態的改變。動物實驗也顯示腸內菌叢可以透過免疫系統與免疫細胞的活化，如巨噬細胞的活化，進而保護呼吸系統，使得肺部免於細菌感染的風險。先前的研究顯示腸道微菌叢對於肺部疾病如氣喘、慢性肺病與肺部細菌感染皆有關連性。自2019年底造成全球肆虐的SARS-CoV-2病毒所造成的COVID-19感染，這種全身性的病毒感染，主要是在肺部出現併發症，愈來愈多的研究顯現消化系統所呈現的徵狀及腸道菌腸道菌群的變化與預後有關，雖然研究結果尚存在分岐，卻是提供腸內菌與肺部感染一些相關的證據。由於腸內菌叢關聯腸道與肺部的健康與疾病狀態，“腸肺軸線”(Gut-lungAxis)概念因而廣泛的被提出引用，除了致病機轉外，在未來肺部感染的治療更提供潛在的重要角色，透過腸內菌叢的保健，對於肺部感染與健康提供更好的治療方案。

The human body is colonized by many kinds of bacteria, which exist in the skin and digestive tract and other organ systems. These numerous bacteria, archaea, fungi and viruses are collectively referred to as microbiota. Parts of these microbiota can affect a variety of physiological and pathological functions through various mechanisms. Nevertheless, pathological condition of host will also manipulate the configuration of the microbial flora. Moreover, many animal models have demonstrated that gut microbiota can protect the respiratory system through the activation of the immune system and immune cells (such as the activation of macrophages), which can further reduce the risk of bacterial infection. The gut microbiota also has pathophysiological role of asthma, COPD and infection. At the end of 2019, the pandemic of COVID-19 infection, which caused by SARS-CoV-2 virus, mainly resulted in pulmonary damages and associated with respiratory complications. Interestingly, the digestive symptoms and gut microbiota are associated with the prognosis of COVID-19 infection, which provided some evidences of the relationship between gut microbiota and lung infections. The concept of “gut-lung axis” is therefore proposed and widely be studied, which not only a new perspective of the interaction but also provide the potential roles of treatment strategies in the future.