After his condition worsened, the doctor decided to have him re-admitted to the hospital.

re-admitted 由前缀 re-（表示"再次"）和动词 admit（"允许进入、接纳"）的过去分词 admitted 组成。re-admitted 表示"再次被接纳进入"，在这里指"重新住院"。 admitted 是 admit 的过去分词形式，在句中构成被动语态，表示"被允许进入"。