His positive attitude was so contagious that the whole team felt motivated.

contagious 来自名词 contagion（传染），加上形容词后缀 -ious 构成。contagion 本身来自拉丁语，表示"接触传染"。 contagious 本义指疾病"会传染的"，引申为情绪、态度等"有感染力的"，形容能像传染病一样迅速影响他人的特质。