He's been plagued by bad luck ever since he bought that house.

plagued 是动词 plague 的过去分词形式，在句中表示被动。plague 本义指"瘟疫、灾祸"，作为动词时表示"使痛苦、折磨"，常用来形容持续不断的困扰或麻烦。plagued by 表示"被...所困扰"。