The custom of saying "God bless you" after a sneeze was begun literally as a blessing. Pope Gregory the Great (540-604 AD) ascended to the Papacy just in time for the start of the plague (his successor succumbed to it). Gregory (who also invented the ever-popular Gregorian chant) called for litanies, processions and unceasing prayer for God's help and intercession. Columns marched through the streets chanting, "Kyrie Eleison" (Greek for "Lord have mercy"). When someone sneezed, they were immediately blessed ("God bless you!") in the hope that they would not subsequently develop the plague. All that prayer apparently worked, judging by how quickly the plague of 590 AD diminished.
The connection of sneezing to the plague is not the first association of sneezing with death. According to Man, Myth, and Magic: The Illustrated Encyclopedia of Mythology, Religion and the Unknown, many cultures, even some in Europe, believe that sneezing expels the soul--the "breath of life"--from the body. That doesn't seem too far-fetched when you realize that sneezing can send tiny particles speeding out of your nose at up to 100 miles per hour!
We know today, of course, that when you sneeze, your heart doesn't stop, nor will your eyes pop out if you can keep them open (
www.straightdope.com/classics/a2_30 4.html), nor does your soul get expelled. What does get expelled are hundreds upon thousands of microscopic germs. The current advice when you sneeze is to cover your mouth with your arm rather than your hand. That way, all those germs won't be on your hands when you touch the countless things you're going to touch in the course of the day (don't tell us; we don't want to know).
There are many superstitions regarding sneezing, some of which you've already listed. But here are some of my favorites.
Sneeze on Monday for health,
Sneeze on Tuesday for wealth,
Sneeze on Wednesday for a letter,
Sneeze on Thursday for something better,
Sneeze on Friday for sorrow,
Sneeze on Saturday, see your sweetheart tomorrow,
Sneeze on Sunday, safety seek.
One for sorrow
Two for joy
Three for a letter
Four for a boy.
Five for silver
Six for gold
Seven for a secret, never to be told.
And lastly, a sneeze before breakfast is a sign that you will hear exciting news before the end of the day.
I assume you sneezed this morning, because you've just been blessed with the Straight Dope.