Sneezing usually occurs when the nerve endings of themucous membraneof the nose are irritated, due to a swelling of the membrane, for example when we have a cold, or when some foreign body, such as a gnat, invades our nose, or when allergy and pollen season strike.
Surprisingly enough, sneezing can be brought on (or out!) when the optic nerves in our eyes are exposed to bright light!
For whatever reason, the membrane is irritated, sneezing is a reflex act, completely beyond our control, by which the nose trumpets out air in an attempt to eject (and project!) the irritating bodies. Sometimes, however, this is accomplished only through multiple sneezes.
Medical science dispelled ancient beliefs concerning the out-of-control sneeze, and snuffed out superstitions in the process. Primitive people held the belief that a sneeze signified approaching death, and immediately assisted the distressed person by crying out "God help you!" Egyptians, Romans, and Greeks, saw the sneeze as an omen of approaching danger, or, on a more positive note, as a way of foretelling the future. Lucky ones sneezed to the right, while unlucky ones sneezed to the left. The moral of the story is to know your right from your left, and to sneeze in that direction, regardless of who is next to you!
Biblically speaking, sneezing meant a certain death, until Jacob nosed in and made a deal with God, whereby a prayer per sneeze cheated the grim reaper. Pope Gregory the Great, in response to the sixth century plague in Italy, carved out his place in history as being the one responsible for insisting that prayers, such as "God bless you!," be said in response to the deadly sneeze. He did not, however, order that tissues be kept close at hand to snare the airborne germs spewed forth by the sneeze.
mucous membrane: 粘膜
（来源： coolquiz.com 英语点津 Annabel 编辑）