1. At death's door
на пороге смерти, на волосок от гибели
My neighbour was at death's door yesterday.
I have fever and cold. I'm feeling like I'm at death's door.
2. Under the weather
нездоровый, больной
Now I am(feeling) under the weather as I have been working continuously for many hours.
You look a bit under the weather.
3. As fit as a fiddle
абсолютно здоровый; в отличной физической форме
fiddle-разновидность скрипки
She as fit as fiddle.
My grandfather is a man of 70. Even at this age he as fit as fiddle.
4. Get back on feet
поправиться, встать на ноги
According to doctor, he will get back on his feet next week.
Hopefully, Odo will get back on his feet soon.
5. Keep in good shape
поддерживать форму, в хорошей физической форме.
It's hard to keep in good shape when you work on computer all day.
I wonder what she does to keep in good shape.