I wondering the same,i dont find anywhere 잘 can mean "easily" ;-;

잘 - well/good/thoroughly/readily

나아요 - to get well/recover

Per the tips and hints in this skill, in Korea a disease recovers from you so 그 병은 나아요 is basically saying "for that disease, one recovers" (i.e. the disease can recover from someone). In English we would say either "that disease is curable/cured" or "one can recover from that disease".

Hence, 그 병은 잘 나아요 is basically saying "for that disease, one well recovers" (i.e. the disease can recover from you easily). In English this would be either "one can recover from that disease easily" or "that disease is easily curable/cured".

Hope that helps somebody.

I think '잘' is where it came from.

For those who don't know, 병 can also mean bottle, and 잘 can be translated as good/well.

아니요, 하지만, "잘"는 Well입니다...