He is dangerously ill.

他病得很重。

《柯林斯英汉双解大词典》

It proved the commencement of delirium: Mr Kenneth, as soon as he saw her, pronounced her dangerously ill; she had a fever.

这证实是热病的开始，肯尼兹先生一看见她，就宣布她病势危险，她在发烧。

youdao

The only son of King Louis VII of France fell dangerously ill and his distraught father took the astonishing decision to go to Canterbury.

法国国王路易七世因为其独子患了重病，变得心烦意乱，遂作出去坎特伯雷朝圣的惊人决定。

youdao

"We think that you ought to have a background check at gun shows. We think you ought to have a background check so that people who are dangerously mentally ill aren't getting their hands on guns."

VOA: standard.2010.04.19

When Mr Yeltsin fell dangerously ill and underwent a heart bypass operation, Mr Chubais held the machinery of government together.

ECONOMIST: The humbling of Chubais | The

An unheralded weekend session of parliament lifted his immunity and he was put in jail, where he remains, despite doctors' pleas that he is dangerously ill.

ECONOMIST: Egypt

Have personnel trained to respond and intervene, possibly to incarcerate the dangerously mentally ill.

FORBES: Searching For Hard Data On Guns And Violence