大病的英文：
a serious illness; a serious disease; a severe disease

参考例句：

Comprehensive arrangement for serious disease
大病统筹

an exhaustive siege of illness
一场耗蚀体力的大病

Not long ago, he was seriously ill.
前不久，他害了一场大病。

“She’s not very ill any more. Console yourself”
“她已没有什么大病，你请放心吧。”

His poor sight is a hangover from a bad illness he had
他视力不好是他生了一场大病留下的后遗症。

serious是什么意思：
adj. 严肃的，认真的；严重的；危急的；重要的；庄重的

It is a serious article.
这是一篇发人深省的文章。

do not deny that is a serious blow.
我不否认那是严重的一击。

It is a job requiring serious application.
这是一项需要非常勤奋的工作。

illness是什么意思：
n. 疾病

Many people are immobilized by the illness.
许多患上这种病的人都瘫痪了。

It becomes ill you to do so.
你这样做是挺不合适的。

They bear you no ill will.
他们对你并无恶意。

disease是什么意思：
n. 疾病，不健全，病害，弊病，烦恼
v. 使有病，使不健全

Surgical disease, or the surgical response to disease is of similar antiquity.
外科疾病或对疾病的外科处理的历史同样久远。

Some diseases are presented by heredity.
有些疾病是由于遗传导致的。

ear bacterial disease of maize
玉米穗细菌病