This illness is fatal in almost all cases.
這種病幾乎是不治之症。
the fatal shooting of an unarmed 15-year-old
對手無寸鐵的15歲少年的致命槍擊
He made the fatal mistake/error of believing what they told him.
他聽信了他們的話,犯了災難性的錯誤。
It just shows how you should never say how well things are going for you - it's fatal (= it causes bad things to happen).
這件事恰恰說明你決不能說事情進展很順利——這麼講會帶來大麻煩。