God of the Merchants, and a Latecomer to Rome

Mercury's caduceus, the herald's staff twined with two serpents and topped with wings, has become one of the most widely recognised ancient symbols in the modern world, though largely through a documented mistake. The medical profession's proper emblem since antiquity is the single serpent wound around the plain staff of Asclepius, the god of healing, a symbol with no connection to Mercury at all. In 1902 the United States Army Medical Department adopted the two-snake caduceus instead, evidently confusing Mercury's herald's staff with the healer's rod, and many American hospitals, ambulances and medical organisations followed that precedent through the twentieth century. The error is now so entrenched in the United States that the caduceus functions as a medical symbol there in practice, even though it properly belongs to the god of commerce, thieves and messengers rather than to healing.

Outside that modern confusion, the caduceus carried a long history as an emblem of neutral status and safe passage, since a herald bearing it was protected from harm even in wartime, and Mercury's association with peaceful negotiation as much as with trade flows from that older, more accurate use of his staff.