This region appears to control the epipharyngeal, intrinsic laryngeal and respiratory muscles, and the combined activity of these muscles serve as the basis for the generation of a sneeze.The sneeze reflex involves contraction of a number of different muscles and muscle groups throughout the body, typically including the eyelids. The common suggestion that it is impossible to sneeze with ones eyes open is, however, inaccurate. Other than irritating foreign particles, allergies or possible illness, another stimulus is sudden exposure to bright light – a condition known as photic sneeze reflex (PSR). Walking out of a dark building into sunshine may trigger PSR, or the ACHOO (autosomal dominant compulsive helio-ophthalmic outbursts of sneezing) syndrome as its also called. The tendency to sneeze upon exposure to bright light is an autosomal dominant trait and affects 18-35% of the human population. A rarer trigger, observed in some individuals, is the fullness of the stomach immediately after a large meal. This is known as snatiation and is regarded as a medical disorder passed along genetically as an autosomal dominant trait. Epidemiology
While generally harmless in healthy individuals, sneezes spread disease through the infectious aerosol droplets, commonly ranging from 0.5 to 5 µm. A sneeze can produce 40,000 droplets. To reduce the possibility of thus spreading disease (such as the flu), one holds the forearm, the inside of the elbow, a tissue or a handkerchief in front of ones mouth and nose when sneezing.
Another less common verbal response in the United States and Canada to anothers sneeze is "Gesundheit", which is a German word that means, appropriately, "good health". Several hypotheses exist for why the custom arose of saying "bless you" or "God bless you" in the context of sneezing:

Some say it came into use during the plague pandemics of the 14th century. Blessing the individual after showing such a symptom was thought to prevent possible impending death due to the lethal disease. In Renaissance times, a superstition was formed claiming ones heart stopped for a very brief moment during the sneeze; saying bless you was a sign of prayer that the heart would not fail. It has also been stated that one says "(God) bless you" so that one does not catch the flu, cold, or any other forms of sickness.Other cultures have similar traditions:

In China, after a person sneezes they often say "百岁!" which translates to "may you live one hundred years!" the pronunciation is similar to "bless you" in English. pronunciation: [Bai Sui]
In Iran, it is common to respond to sneezing with the Persian phrase عافیت باشه âfiyat bâše, which translates to "health", similar to common European expressions. Indian culture is to respond with Krishna, similar to a blessing in western cultures. In Italy after a person sneezes the people present respond with the word "salute" (meaning: health). The louder the sneeze the more emphatic the response. In Slovakia, after a person sneezes, it is proper to say "Na zdravie!" which means "For health!