We didn't fully appreciate that he was seriously ill.

我们没有充分认识到他的病情很严重。

《牛津词典》

She was seriously ill as an infant.

她年幼时曾患重病。

《牛津词典》

He was later discovered to be seriously ill.

后来才了解到他患了重病。

《牛津词典》

We will go back before you get seriously ill.

VOA: special.2009.02.07

And he plans to help others who are seriously ill.

VOA: standard.2010.01.12

More than half the patients with confirmed infection have died and others remains seriously ill.

BBC: Threat from novel coronavirus 'remains low'

Around 700 people had to be tested for radioactive poisoning but none was seriously ill.

BBC: Who was Alexander Litvinenko?

She then received terrible news: her mother had developed gangrenous bed sores, and was seriously ill.

BBC: NEWS | Health | 'I wish I could have done more'