Why do people say 'God Bless You' when you sneeze?
There are several theories for this.
One theory says that the phrase "God Bless you" came from an
Islamic practice which originated over 1400 years ago. According to
the Islam religion, when someone sneezed, he should say "All
Praises to God" and in response the people would reply "May God
Bless You".
Another explanation is that when people sneezed it was believed
that the heart would miss a beat; in that microsecond, the skip
would allow the devil to enter the body. Therefore, saying "God
bless you" would stop the devil from entering one's body
uninvited.
Another middle ages superstition held that, when one sneezed, a
large amount of breath (regarded as the very breath of life) could
be expelled suddenly from the body, resulting in death. In case the
victim died in this instance, he/she would at least go to heaven
with God's Blessing.
Written records state that the saying goes back to the time of
Pope Saint Gregory I, or Gregory the Great, who was Pope of the
Catholic Church from September 590 until he died in 604. When Pope
Gregory ascended to the Papacy, it was just in time for the start
of the Plague, so this Pope is unfortunately known as the patron
saint of plague. He believed that constant repetition of litanies
and unceasing prayer for God's help and intercession would help
ward off sickness. On 16 February 590 A.D., Pope Gregory decreed
that whenever someone sneezed, others should say "God bless you" in
response. The blessing was given in the hope (or belief) that the
one who sneezed wouldn't then develop the plague.
Perhaps there was more to this than people realised: it is
interesting to note that the plague of 590 A.D. dissipated very
quickly.