Here are some idioms to do with health.

To say that you are in good health, you can use the expressions 'as fit as a fiddle' or 'fighting fit'.

• I've never felt better. I'm really fighting fit.
• After those vitamins the doctor gave me, I feel as fit as a fiddle.

(Yes, 'fiddle' is another word for 'violin'. No, I don't understand the origins of this expression either!)

To say that you are OK ( but are not in really good health) use 'I can't complain'

• I've had a bit of a cold but I can't complain.

(As I'm still off work, I've not been able to check out if this expression is used in US English. I'll let you know later.)

To say that you have recovered from an illness, you can use these expressions:

• I was knocked out for a few days but now I'm back on my feet.
• I was quite ill but now I feel as right as rain.
• I'm well on the way to recovery.
• The doctor gave me a clean bill of health.

Here are some expressions to indicate you are not in good health.

• I'm a bit out of sorts.
• I'm going down with something.
• I'm feeling very run down.
• There's a bug going round and I think I've got it.
• I'm feeling a bit off-colour/off-color.
• I'm feeling under the weather.

Here is an expression for bad headaches.

• I've got a splitting headache.
  
  

Here is an expression to indicate a lot of pain.

• My legs are killing me.
• My back is killing me.

Here is an expression to indicate a serious illness.

• He's in a very bad way.