An apple a day keeps the doctor away

Apples have a good claim to promote health. They contain Vitamin C, which aids the immune system, and phenols, which reduce cholesterol. 

They also reduce tooth decay by cleaning one's teeth and killing off bacteria. 

It has also been suggested by Cornell University researchers that the quercetin found in apples protects brain cells against neuro-degenerative disorders like Alzheimer's Disease.

Apples may be good for us but it wasn't their precise medicinal properties that were being exalted when this phrase was coined. 

In Old English the word apple was used to describe any round fruit that grew on a tree.