The onset of something is the moment it begins — and English almost always keeps this word for things we do not welcome. The onset of winter, the onset of war, the onset of a fever.

The onset of something某件事的 onset是它開始的那一刻 is the moment it begins — and而且 English almost always keeps this word for things we do not welcome英文幾乎都把這個字留給不受歡迎的事情用 the onset of winter像 the onset of winter（冬天來臨）、 the onset of war the onset of war（戰爭爆發）、 the onset of a fever the onset of a fever（發燒發作）

Doctors especially like it: sudden onset means it started quickly; slow onset means it crept in.

Doctors醫生 especially like it特別喜歡用這個字 sudden onset sudden onset意思是突然發作 means it started quickly開始得很快 slow onset slow onset means it crept in則是慢慢潛入、逐漸出現

It is a fairly formal, textbook-and-news word. In daily talk a student would more often say when it started.

It它 is a fairly formal, textbook-and-news word是個相當正式、常見於教科書和新聞的字 in daily talk日常對話裡 a student學生 would more often say when it started比較常會說 when it started（它是什麼時候開始的）

There is an old verb to onset, meaning to attack or to press down on someone, but almost nobody uses that now; if you meet it, it is in old writing. The noun is the one you need.

There is有 an old verb一個古老的動詞 to onset to onset meaning to attack or to press down on someone意思是攻擊某人或壓制某人 but almost nobody uses that now但現在幾乎沒人這樣用 if you meet it如果你遇到 it is in old writing那會是在古老的文章裡 the noun名詞用法 is the one you need才是你需要的

You can say at the onset, which means at the very beginning, still usually of something serious.

You你 can say at the onset可以說 at the onset which means at the very beginning意思是在最開始的時候 still usually of something serious通常還是指某件嚴重的事