(as) fit as a flea

What is the origin of the idiom "fit as a flea" and when to use it?

The phrase "fit as a flea" is a variation of the older phrase "fit as a fiddle," which originated in the 17th century. The comparison to a flea suggests a small and agile creature that is known for its energetic and active nature. It is used to describe someone who is in excellent physical health, vitality, and overall condition.