deadly 普通用詞，指極為可能致命的或致死的。
1）A single leakage of a radioactive substance, however tiny, can be deadly so
extreme caution must be exercised.
放射性物質泄漏, 無論多麽少量，都可能是致命的，因此必須高度謹慎。
2）The cyclist was knocked down by a lorry and received a deadly injury.
騎車人被壹輛卡車撞倒，受了致命的傷。
fatal 指可能導致或已經導致死亡的，也可表示壹事對另壹事是毀滅性的或災難性的。
1）Diarrhea is one of the most common fatal childhood dieseases worldwide.
痢疾是世界範圍內兒童常見致死疾病之壹。
2）The event proved to be fatal to his political future.
這次事件對他的政治前途證明是致命的。
lethal 只用於指確定能致死的事物，如lethal gas (毒氣)、lethal dose (致命劑量)。
1）Health authorities fear that this strain could cause a new lethal flu pandemic.
衛生部門擔心這種菌株會引起新的致命流感流行。
2）Dozens of people queue in hot sun outside to return milk powder that could be
contaminated with a potentially lethal chemical.
有幾十人在炎熱的太陽外面排隊要求退回被化學物質汙染的可能引起死亡的奶粉。
mortal 正式用詞，用法類同fatal，可換用。mortal 的名詞是 mortality，較death正式，常指“死亡率”或“死亡數目”。
1）He got a mortal（fatal）blow on the head and died soon after.
他頭部被人致命壹擊,不久便去世了。
2）The new device has a mortal （fatal） flaw: it will cease functioning at an
extremely high temperature.
這種新裝置有壹個致命的缺陷，在極高溫下會停止運行。
3）Infant mortality（death rate）is still very high in some developing countries.
某些發展中國家的嬰兒死亡率仍然很高。
4）Particulate air pollution is closely associated with cardiovascular mortality
and morbidity.
微塵空氣汙染與心血管疾病的發病率和死亡率密切相關。