This disease is one of the most common infectious diseases , and a serious problem.

这种病是最常见的传染病之一，这是个严峻的问题。

youdao

In 'Luckily, I didn't tell anyone' and 'Financially, we have a serious problem', 'luckily' and 'financially' are sentence adverbs.

在句子Luckily, I didn't tell anyone和Financially, we have a serious problem中，luckily和financially是句副词。

《牛津词典》

He has recently made a significant discovery—loneliness is a serious social problem that can contribute to depression and even crimes, but it can be prevented in a clever way.

他最近有了一项重大发现——孤独是一个严重的社会问题，它会导致抑郁甚至犯罪，但可以用一种聪明的方法加以阻止。

youdao

Therefore the availability of official documentation for sale to the highest bidder is a serious problem and deserves serious attention.

CENTERFORSECURITYPOLICY: Center For Security Policy

And while it is a serious problem that must be addressed, rebuilding the economy and raising employment and real wages is a great way to pay down that debt.

FORBES: False Assumptions in Obama's Deficit-Reduction Plan

The spread of weapons of mass destruction is a serious and actual problem and vigilance is important.

CENTERFORSECURITYPOLICY: Blowing Smoke About Making Deadly Gas