I found this interesting article about the expression “Bless you!” on an American site. The link to the full article is just after the text, in case you want to learn more …
Many people have become accustomed to saying “bless you” or “gesundheit” when someone sneezes. No-one says anything when someone coughs, blows their nose or burps, so why do sneezes get special treatment? What do those phrases actually mean, anyway?
Wishing someone well after they sneeze probably originated thousands of years ago. The Romans would say “Jupiter preserve you” or “Salve,” which meant “good health to you,” and the Greeks would wish each other “long life.” The phrase “God bless you” is attributed to Pope Gregory the Great, who uttered it in the sixth century during a bubonic plague epidemic (sneezing is an obvious symptom of one form of the plague).
The exchangeable term “gesundheit” comes from Germany, and it literally means “health.” The idea is that a sneeze typically precedes illness. It entered the English language in the early part of the 20th century, brought to the United States by German-speaking immigrants.