An apple a day keeps the Doctor away.

[ChamakC]

When a person is sick or ill, they normal take medicine or special table to cure their sickness.
So how could an apple a day keep the Doctor away ?

[SMILL48]

Apples are one of the most useful remedies and have been used for centuries. They're a great digestive aid, and help to detoxify the body. Grated apple, for example, is a remedy for diarrhoea. Research shows that people who eat more apples have fewer colds, and lower cholesterol levels. Apples also have anti-viral and anti-cancer properties!

[Piljim]

As the last answer said, apples have been demonstrated in some research to prevent colds. This is possibly due to the Vitamin C (ascorbic acid) content - a natural antioxidant that can prevent damage to tissues etc. Oranges and other citrus fruits are better sources of Vitamin C.

[CB100]

I have been eating two apples a day for years and now my dentist tells me that the fruit acid in them is the cause of my receeding gums and the eroding enamel on my teeth...

[Ginbottle]

The period in history when this phrase started, the fresh apples would have kept away scurvey; especially through the winter months when very few other fresh fruit or veg was available.