I used to think it was from the Greek God Mercury’s staff, but I was wrong!
So from Wikepedia (http://en.wikipedia.org/wiki/Rod_of_Asclepius):
In Greek mythology, the Rod
of Asclepius, sometimes also
spelled Asklepios or Aesculapius, also known as the asklepian, is a serpent-entwined rod wielded by the Greek god Asclepius, a deity
associated with healing and medicine. The symbol has continued to be used in
modern times, where it is associated with medicine and health care, yet
frequently confused with the staff of the god Hermes, the caduceus. Theories have
been proposed about the Greek origin of the symbol and its implications.
In honor of
Asclepius, a particular type of non-venomous snake was often used in healing
rituals, and these snakes – the Aesculapian snakes –
crawled around freely on the floor in dormitories where the sick and injured
slept. These snakes were introduced at the founding of each new temple of
Asclepius throughout the classical world. From about 300 BC onwards, the cult
of Asclepius grew very popular and pilgrims flocked to his healing temples
(Asclepieia) to be cured of their ills.
The original Hippocratic Oath began
with the invocation “I swear by Apollo the Physician and by Asclepius and
by Hygieia and Panacea and by all the gods …”
But why a snake? The significance of
the serpent has several interpretations:
and renewal is emphasized as symbolizing rejuvenation
and expresses the dual nature of the work of the physician, who deals with life
and death, sickness and health.
harm, as reflected in the meaning of the term pharmakon, which meant
“drug”, “medicine” and “poison” in ancient Greek. Products derived from snakes had medicinal
properties, and in ancient Greece, at least some were aware
that snake venom that might be fatal if it entered the bloodstream could often
be imbibed. Snake venom appears to have been ‘prescribed’ in some cases as a
form of therapy.