A sudden onset of intense physical symptoms or a paroxysmal medical episode—used for seizures, spasms, fits of coughing, epileptic attacks, etc.
彼はてんかんの発作を起こして倒れた。
He had an epileptic seizure and collapsed.
激しい咳の発作で夜眠れなかった。
I couldn't sleep at night because of severe coughing fits.
発作が起きたらすぐに救急車を呼んでください。
If an attack occurs, call an ambulance immediately.