The doctor recommended immediate treatment, so he was quickly admitted to the hospital.

admitted 是动词 admit 的过去分词形式，admit 的意思是"允许进入、接纳"。在医疗语境中，admit to the hospital 特指"办理住院手续、收治入院"。 这里用被动语态 was admitted 表示"被收治入院"的意思。