The outbreak was considered an isolated event and was quickly contained.

an 是不定冠词，用于表示单数可数名词的泛指。 isolated 是形容词，来自动词 isolate（隔离、孤立），意思是"孤立的、单独的"。 event 是名词，意思是"事件、事情"。 isolated event 指"孤立事件"，表示与其他事件没有关联的单一事件。