China knew about the contamination of milk products months ago, but covered the scandal up to prevent it tarnishing the Beijing Olympics.
中国は乳製品の（有害物質メラミンによる）汚染を数ヶ月前に知ったが、スキャンダルとなって北京五輪を傷つけるのを防ぐため隠蔽した。
◆cover up ： 隠す、隠蔽する
◆tarnish ： 名誉を損なう
LONGMAN's definition
★[Cover sth ⇔ up] to keep mistakes or unfavorable facts from being known about
(ex.)Christmas can cover up some of their illegal activities by dealing only in cash.
---see also cover-up ⇒an attempt to prevent the public from discoverng the truth about something