He contracted a serious illness during his travels.

contracted 是动词 contract 的过去式。contract 由前缀 con-（表示"共同"）和词根 tract（拉、拖）组成，原意是"拉到一起"，引申为"收缩、缩小"。在医学语境中，contract 特指"感染、患上（疾病）"，表示疾病与身体产生了联系。