从流行病学的角度来看，偶然暴发一般存在有散在病例的背景。

In epidemiological terms, there is generally a background of sporadic cases, with occasional outbreaks.

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第二种是偶然读点书的人。

The second group are the Occasional Readers.

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只是偶然有载满蔬菜的货车开过边境。

Only the very occasional lorry laden with vegetables trundles over the frontier.

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In the first place, a mask is nearly always occasional.

首先，这个节目几乎是偶然的。

耶鲁公开课 - 弥尔顿课程节选