"health"和"exercise"是可数名词还是不可数名词?

"health"和"exercise"是可数名词还是不可数名词?
1.health是可数名词还是不可数名词?
可数理由：She is in a good health.为什么用"a"?
不可数理由：We should look after our health?为什么不是"healths"?
2.exercise（意为“锻炼”时）是可数名词还是不可数名词?
可数：You had better do morning exercises.
不可数：练习册上说,exercise作“锻炼”时可以是动词,也可以是不可数名词.You should take more exercise.
2.exercise
很多地方说，exercise作名词时不可数，如take more exercise/do exercise，但也有do morning exercises,为什么？