People who have contracted serious diseases often require specialized treatment.

contracted 是动词 contract 的过去分词形式。contract 在这里的意思是"感染（疾病）"，指病原体进入人体并引发疾病的过程。这个词来源于拉丁语，原意是"拉在一起"，引申为"获得、染上"的意思。