Why do they say 'God Bless You' when you sneeze?

The practice of saying ‘Bless You’ when someone sneezes owes its origin to a practice
which was started by Pope Gregory I (AD 540 to AD 604) in the year AD 590.
The legend goes that an epidemic called the ‘Bubonic Plague’ broke out and Pope Gregory I ordered for worship sessions, processions and unceasing prayers for God’s blessings in order to combat the disease. One of the major symptoms of the disease was sneezing. When someone sneezed, he was almost sure to get affected with plague soon.

During the prayers and the gatherings, whenever someone sneezed, they said ‘God Bless You’, so that he doesn’t end up getting the disease. Thus, the practice of saying ‘Bless You’ or ‘God Bless You’ came into existence.Another popular explanation for the practice is that, when a person sneezes, his heart stops for a fraction of a second. They say ‘Bless You’ to wish that the heart starts beating again and continue to function.

image source: wikipedia