Saying "bless you" is a way of wishing a person good health, and the custom began thousands of years ago. The Romans said "Jupiter preserve you" or "Salve," meaning "good health to you." The phrase "God bless you" is attributed to Pope Gregory the Great, who, during an outbreak of bubonic plague, instructed his followers to use the blessing as a way of warding off the disease.
The origin of the custom of blessing someone when he sneezes is unknown, but probably stems from the idea that a sneeze usually precedes an illness. The German version of the blessing, "gesundheit," literally means "health," and to say it is to wish someone good health. The custom may also have originated as a response to any number of superstitious beliefs about the nature of sneezing, such as the idea that a person's heart stops when they sneeze or that someone can expel their soul from their body with a sneeze. Whatever the reason, the custom is established throughout the world. In Arabic countries, people say "Alhamdulillah," which means "Praise be to God." The Hindus exclaim "Live" or "Live well!" In Spain, they say "Salud!" In China, when a child sneezes he is told "bai sui," which means "may you live 100 years."