The U.S. Army Medical Corps chose the caduceus as its symbol in 1902.
“Since then the politics of health care have grown more twisted and tangled than the two snakes entwined around the staff in a caduceus, which is sometimes used as a symbol of medicine.” — From an article by Michael Cooper in The New York Times, February 15, 2012
The Greek god Hermes, who served as herald and messenger to the other gods, carried a winged staff entwined with two snakes. The staff of Aesculapius, the god of healing, had one snake and no wings. The word ''caduceus,” from Latin, is a modification of Greek “karykeion,” from “karyx,” meaning “herald.” Strictly speaking, “caduceus” should refer only to the staff of the herald-god Hermes (Mercury to the Romans), but in practice the word is often applied to the one-snake staff as well. You might logically expect the staff of Aesculapius to be the symbol of the medical profession—and indeed, that is the symbol used by the American Medical Association. But you will also quite frequently see the true caduceus used as a medical symbol.
From Merriam-Webster Online at www.Merriam-Webster.com.