Why did benevolent wishes like "Bless you" or "Gesundheit" attach themselves inseparably to the sneeze? No routine comment is invited by someone belching, coughing, groaning, hiccuping, retching, snoring, vomiting, wheezing, or breaking wind, even when these symptoms may portend trouble. A reflex surviving all of the ancient omens and firmly established in folklore demands an explanation.
Centuries of such popular usage would make it appear that the solicitous expressions may have been generated so as to ward off an ominous threat. Moreover, lodged in the sneeze is a protective reflex—the automatic closing of the eyes. Even the cough, most closely allied physiologically,1 can be accomplished with the eyes open. (In common with the cough, however, the sneeze can spread disease.) These observations suggest a danger inherent in the sneeze.
The sneeze has different meanings in different cultures.2 Romans, following Persian demonology, responded to the sneeze with Absit omen!
Kavka SJ. The Sneeze—Blissful or Baneful?. JAMA. 1983;249(17):2304-2305. doi:10.1001/jama.1983.03330410018008