For the former, they will not be on the alert, while the latter is often scary.

对于前者，他们不会加以提防，而对后者却常常提心吊胆。

youdao

They were constantly on the alert not to be taken by surprise.

他们一直处于常备不懈状态以防突然袭击。

《新英汉大辞典》

But I insist that the virus is still present, that we need to remain on alert and the resumption of activities will be little by little, not all at once.

但是我坚信流感现在仍然活跃，我们需要继续提高警惕，病毒的反扑是一点一点体现出的，而不是一下子爆发。

youdao

Investors now know that funds are not inherently safe and that they should always be on the alert for self-dealing by advisers, brokers and other intermediaries.

FORBES: The Great Mutual Fund Clean Up That Almost Happened (April 4, 2004 )

That doesn't mean the Transportation Security Administration, the airlines and airports won't be on high alert, although they are not communicating many details of increased security to the public.

CNN: Who's afraid to fly on September 11?

The government said it was acting on intelligence information, but Home Secretary David Blunkett said the public should be "alert, not alarmed".

BBC: Osbourne drops UK visit over terror fears