"Bless You" 

 People say "Bless you when someone sneezes because the various of sneeze responses origniated for superstitions. Some people beleived that sneezing caused the soul to leave the body throught the nose. 

By saying "bless you" it would stop the devil from claiming one's soul 

This saying originated a very long time ago, there's a lot of interpertations of why we use this phrase. But It was literally meant as a blessing. 

Pope 1 the great (540-604 AD)  used "God bless you" whenever someone sneezed becasue that was one of the symptoms in the plague so he wold bless them in hope they didnt get the bubanic plague,

The practice of blesing someone after they sneeezed dates back to AD 77.