急性 jíxìng acute

怎么记住“急性”: 急 urgent + 性 nature = acute.

汉字

Medical/technical modifier for "acute" (fast-onset) illness - 急性肠炎 (jíxìng chángyán) acute enteritis, 急性中毒 (jíxìng zhòngdú) acute poisoning.
Its antonym is 慢性 (mànxìng, chronic); the 急性/慢性 pair is the standard fast-vs-slow split for diseases - 慢性病 (mànxìngbìng) a chronic illness.
Also describes a hot-tempered personality: 他是个急性子 (tā shì ge jíxìngzi) he's an impatient sort - note that sense uses the fixed word 急性子, not bare 急性.