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!
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.
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...
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.