At one time people believed that an explosive sneeze could thrust your soul out of your body. Saying "bless you" was protection.
Bless you, or God bless you, is a common English expression typically addressed to a person after they sneeze. Alternative responses to sneezing adopted by English speakers are the German word gesundheit (meaning "health"), the Irish word sláinte (meaning "good health"), and the Spanish salud. The origin of this custom and its original purpose are unknown.
Several possible origins are commonly given. The practice of blessing someone who sneezes, dating as far back as at least AD 77, however, is far older than most specific explanations can account for. Soul
In English-speaking countries, the common verbal response to another person's sneeze is "(God) bless you" or, less commonly in the United States "Gesundheit" (good health). There are several proposed origins for the use "Bless you" in the context of sneezing.
In non-English-speaking cultures, words referencing good health or a long life are often used instead of "bless you", though some also use references to God. Physiology