a.

1. 传染的；传染性的
   an infectious disease

   传染病

2. 有感染力的，易传播的
   an infectious grin

   有感染力的笑

   Their enthusiasm was infectious.

   他们的热情有感染力。

3. 【废】感染疾病的

adj.

1. (of a disease or disease-causing organism) liable to be transmitted through the environment.
   ▸liable to spread infection.
2. likely to spread to or influence others:
   her enthusiasm is infectious.