It all stems back to the days of the Great Plague of London (1664-1665), people were always looking for a sign of the plague in themselves and others and they decided that sneezing was an early indication.
So, when someone sneezed, they would immediately say, ‘God bless you’ or ‘Bless you’.
There is also a link between sneezing and the plague found in the nursery rhyme ‘Ring a Ring o’ Roses, which contains the line: Atishoo, atishoo, we all fall down’ – or ‘sneeze, sneeze, we all drop dead’.
The other interesting fact is that every time we sneeze, our hearts stop for a fraction of a second!