Related article

The symbol of the World Health Organization, the American Medical Association, and most professional medical associations worldwide is a staff or rod with a single snake wrapped around it. Called the staff of Asclepius, this is the traditional symbol of the medical profession. However, in the United States, the Public Health Service and some other American medical bodies use as their symbol the caduceus, a shorter stick entwined by two serpents and topped with two wings. The caduceus is actually the magical wand of the Greek messenger god Hermes (the Roman Mercury). Why are two similar (yet different) symbols used?
 
 

Asclepius was the Greek god of healing. His staff was a stick with a snake coiled around it.

The caduceus was the wand of Hermes, the Greek messenger god. Alchemists adopted it as their symbol of magic. However, they also mixed drugs to treat illnesses, and the symbol’s meaning may have gotten mixed up because of the different jobs of the alchemist.

The association of snakes and healing is an ancient one. Snakes were part of cult practices in Mesopotamia, in Egypt, and in the Minoan culture of Crete. There is another wrinkle to this tale, though. Some medical historians believe that the creature adopted as the symbol of medicine is not a snake at all. They suggest that it may be a guinea worm, a parasite common in warmer parts of the world. The guinea worm lives just under the skin and can grow to 1 m (3.3 ft) in length. Ancient physicians knew how to remove these worms. They would cut a slit in the skin just in front of the worm and encourage it to wrap itself around a stick as it very slowly exited the body. This delicate process, which takes many days, is still sometimes used in Africa, where guinea worms remain a serious health problem. Most scholars identify the "fiery serpents" that beset the Israelites as they fled Egypt (as described in the Bible’s Book of Numbers) as guinea worms. And what happened? According to the Bible, God told Moses to create a brass serpent and mount it on a pole so that anyone who looked at it would be cured. Whether it is a snake or a worm wrapped around the stick, this is an ancient image.
The first true association of a snake wrapped around a pole with the art of healing comes from ancient Greece. Asclepius was the Greek god of healing. As the half-mortal son of Apollo, he is said to have learned some of his skills from seeing a snake that he had killed with his staff revived by another snake that stuffed herbs into its mouth. He then used the same herbs to bring back to life a man killed by a thunderbolt from Zeus. Fearing that Asclepius might make all humans immortal, Zeus then used another thunderbolt to kill him. Later Apollo prevailed on Zeus to make Asclepius a god, the god of medicine.
This is all myth, of course. But Asclepius was worshipped as the god of medicine. There were temples or shrines erected in his honor all over the Greek world, and people went to them seeking cures. At the Asclepions, as they were called, patients slept on the floor while harmless snakes slithered among them. Ancient depictions of Asclepius show him with that staff and the single serpent.
So where does Hermes and his wand come into it? Sometime during the Middle Ages in Europe, the caduceus, a symbol of magic, became associated with alchemists. Alchemy, an ancient tradition found in Asia as well as Europe, blended science (particularly chemistry) with the occult in a complex system of philosophical and religious beliefs. Alchemists prepared drugs, however, and so were a part of traditional medical practice. This may be one source of the confusion.
More simply, though, one19th-century English printer of medical texts used the caduceus as his printer’s mark, or trademark. For him it was a symbol of commerce, since Hermes was also the god of commerce. However, an American army officer understood it to be a medical symbol and lobbied hard to get it adopted as the symbol of the U.S. Army’s Medical Corps. This happened in 1902. That was the beginning of its fairly wide-scale adoption in the United States. Today both symbols are recognized for their association with the medical profession.