Why do many Western people say these words after someone has sneezed? As with many things the origin is so old that it is unclear, though most people agree that it means something like, ‘May God bless you‘, so it must be concerned with something very important.
Some people think that a sneeze causes the soul to be blown out of the body. The words ‘Bless you’ are used to stop Satan (the devil) from stealing the soul, before it can return to the body.
Other people believe the sneeze blows the devil from the body, and ‘Bless you’ prevents the devil from returning to the body.
It was also thought that the heart stops beating for a few moments when someone sneezes (don’t worry, it doesn’t), so the words ‘Bless you’ can help the person return to life.
But the reason most people believe is related to a disease known as the ‘Black Death‘ which killed thousands, perhaps millions, of people in Europe around the 17th century. If someone caught this disease then he would start sneezing. So a sneeze was a sign that this person would soon die, and the words ‘Bless you’ were said as only God could now help this person.
Nowadays it seems that ‘Bless you’ or ‘God bless’ is a standard response to someone who sneezes, just good manners. Staying silent after someone has loudly sneezed seems impolite and unfriendly.
But there is one final point: if your friend suffers from hay-fever you don’t need to say ‘Bless you’ after every sneeze. In this case I am sure your friend would be quite happy if you simply continued with your conversation.
thiNK TWice 
‘Bless you’ make me think about a Japanese expression ‘okage-sama-de’. It’s often quoted as how unclear Japanese language is.
‘Okage-sama-de’ is probably translated into ‘I/We owe (my/our wellbeing)’: to what or to whom does the person saying this word owe, it is left unsaid. Maybe that’s “you” or divine intervention, maybe all the existence outside of the person.
Recently I’ve been wondering this not-telling-the-detail speech is not the monopoly of Japanese. You explained us about “bless you” and encourage my reasoning. “It” in these usages is not quite clear:
It’s fine.
It rains a lot.
It’s hot today.
We made it.
I like it when you are smiling.
I have hard time explaining the meaning of “it” in these usages to my students. And now, they seem to refer to the same something we omit mentioning when we say “okage-sama-de”.
Am I digging it too much?
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I would concur with Tom. This is also why people are requested to hold their hand in front of their mouth and if possible keep their mouth shut when sneezing, coughing, yawning and even sniffing.
In my country, France, it has been solidly proved that it lost over 35% of its population within two years during the second wave of bubonic plague in the 15th Century.
Did you know that the real meaning of “Renaissance” was the rebirth witnessed after the last plague in the 15th Century?
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Thanks for the correction.
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I think you’re confusing the Great Plague of London mid 17th century with the Black Death of the 14th century. The Black Death is thought to be the origin of ‘Bless you’ and also of the children’s song ‘Ring a Ring a Rosies …….’. Some experts put the death toll in England at around 70%.
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