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.