流感爆发后，学校暂时停课了。

The school was temporarily closed after the flu ___outbreak.

社交媒体上突然爆发的讨论让这个话题迅速走红。

The sudden ___outbreak of discussion on social media made this topic go viral quickly.

抗议活动的突然爆发让政府措手不及。

The sudden ___outbreak of protests caught the government off guard.