infectious 的用法及固定搭配

contagious disease是“跟病人直接或间接接触而传染的病”，infectious disease是“通过空气、水等传染的病”。infectious disease未必是contagious disease。但说“感染性的”或“蔓延的”contagious和infectious很少区别。例如：Laughter is contagious和Laughter is infectious都可以说。参见contagion