Usually, the virus is not serious and only lasts a few days.

通常，这种病毒不严重，只持续几天。

He is always not serious about his work.

他对待工作总是不认真。

The doctor assured her that her condition was not serious.

医生使确信她，她的病情并不严重。

How can you succeed if you are not serious like this?

你这样不认真，怎么能成功呢？

He is always not serious about life, so problems often arise.

他对待生活总是不认真，所以常常出问题。

If you are not serious about this matter, you will have trouble.

如果你不认真对待这件事，会有麻烦的。

He was just engaging in a flirtation with her, not serious at all.

他只是和她逢场作戏，并不认真。

Her attitude towards studying is not serious, so her grades are naturally poor.

她对学习态度不认真，成绩自然不好。

The patient's disease was not serious enough.

病人的病情还不够严重。

I'm not serious. You're the person who's serious, Joker!

我不严肃。你才是那个严肃的人，小丑！