In many Western countries, and in movies and TV you may notice an odd custom. Whenever someone sneezes someone nearby will usually say “Bless you”, or “God bless you” or “Gesundheit”.
This is a strange custom, but it is considered polite, and people will often offer gratitude.
…is a very common exchange to hear.
This custom comes from long ago when people believed that whenever you sneezed the devil had a chance of entering your body. So after someone sneezed, people thought that saying “God bless you” would cast the devil out of their bodies. Naturally others would thank that person for saving them from the devil.
We no longer believe in that mythology, but it is still considered polite to say something when someone sneezes. At least for the first sneeze or two. Some people might even consider you rude if you don’t.
“Bless you” and “God bless you.” are both based on the original myth of casting out demons. “Gesundheit” is a German word that means “Health” that people sometimes say.
Since sneezing sometimes indicates that someone is sick, saying one of these things is just a way to show concern for other people. Even strangers. Don’t be surprised if an English speaker says it to you!