@xianc78 i've never heard that theory before. i've always heard that it popped up because of the plague so people would say "bless you" to give them a blessing of health.
if someone sneezes today, the same thing applies. you are blessing them for their health.
@beardalaxy If that's the case then people should be saying "bless you" when someone coughs, and coughing is much more dangerous than sneezing.