why do people say bless you when you sneeze

People say "bless you" when someone sneezes mostly due to historical and superstitious reasons. One common origin traces back to the Middle Ages during the bubonic plague when sneezing was a symptom, and the phrase "God bless you" was a prayer to protect the person from death. Another belief is that a sneeze might expel the soul from the body, so saying "bless you" was meant to safeguard the soul from evil spirits or the devil. Today, it's mostly a polite social custom without serious superstition behind it. In some cultures, other expressions for health and well-being are used, such as "Gesundheit" in German, meaning "health".