A: You look unwell. Why don’t you go home?
B: Don’t worry.
A: But you must not get ill. It’s even getting cold.
B: I shall not die from indisposition. I have to finish it by the end of the day.
In this situation, what should you say ‘but you must not get ill’ in Japanese?
Casual
A：でも、病気になったら大変だよ。
Note: This is not a word-for-word translation.
Advanced Tips
A word-for-word translation
A：でも、病気になってはいけません。
It’s grammatically correct but we don’t usually say it in this context.
でも、病気になったら大変 is a common expression.
大変 is a handy word and you can put a lot of meaning into it.
In this situation, you can imply that you’re worried about them, that you don’t agree with them not going home, and even that people around them would be bothered.
It also helps make them to imagine the consequences of staying there.
Kotoba no Hibiki 