contagious是指經接觸性傳染,如:
The infection is highly contagious, so don't let anyone else use your towel.
而infectious是指:可以播染的(沒有指定的途徑),如:
Flu is highly infectious.
而epidemic,是指:流行病;有廣泛傳染性的(指一種疾病,同時傳染許多人),如:
Over 500 people died during last year's flu epidemic