How to say "outbreak" in Japanese

• 発生はっせいJLPT N1approx.Common
  noun, suru verb, intransitive verbCovers outbreak in the neutral sense of something occurring or arising, such as the incidence of disease or trouble. It is the broad, everyday choice, and also extends to occurrences like the birth of civilization.
• 突発とっぱつCommon
  noun, suru verb, intransitive verbCovers outbreak as a sudden, unexpected occurrence. It emphasizes that something breaks out abruptly, and is used for occurrences of emergencies or unforeseen incidents rather than ordinary, expected events.
  その突発とっぱつについて不必要ふひつように心配しんぱいする必要ひつようはない。↗

  There is no need to be unnecessarily anxious about the outbreak.
• 勃発ぼっぱつCommon
  noun, suru verb, intransitive verbCovers outbreak in the sense of war or similar conflict suddenly breaking out, as well as outbursts. It is really the word for the sudden eruption of war or confrontation, not for general occurrences.
  第だい一いち次じ世界せかい大戦たいせんは1914年ねんに勃発ぼっぱつした。↗

  World War I broke out in 1914.

Use 発生 for outbreaks in the neutral sense of occurrence or incidence, 突発 for sudden unexpected occurrences, and 勃発 specifically for the outbreak of war, conflict, or other violent eruptions.

Usage comparisons are AI-assisted and cross-checked by multiple models; for study reference only

• 発生はっせいoccurrence
• 突発とっぱつoutbreak
• 勃発ぼっぱつoutbreak (e.g. of war)
• アウトブレイクoutbreak (esp. of a disease)
• 発災はっさいoccurrence of a disaster
• 発来はつらいonset
• オーバーシュートexplosive spread (of an infectious disease)