The doctor warned that even after recovery, you could still be reinfected with the flu virus if not careful.

reinfected 是动词 reinfect 的过去分词形式。reinfect 由前缀 re-（表示"再次"）和动词 infect（感染）组成，意思是"再次感染"。 be reinfected 是被动语态结构，表示"被再次感染"。