Why do we say Bless You when someone sneezes? It is habit or plain superstition. I did a bit of research on about sneezing and it all stemmed from the great plague in 6th century.
National Geographic reports that during the plague of 590 AD, “Pope Gregory I ordered unceasing prayer for divine intercession. Part of his command was that anyone sneezing be blessed immediately (“God bless you”), since sneezing was often the first sign that someone was falling ill with the plague.” By 750 AD, it became customary to say “God bless you” as a response to one sneezing.
National Geographic : From Piazza San Pietro, proceed down the broad avenue across from the Basilica, Via della Conciliazione (commissioned by Mussolini to add grandeur to the site), to (4) Castel Sant’Angelo (Lungotevere Castello 50), a round-walled, battlemented structure that today serves as a museum. Commissioned as a mausoleum for Emperor Hadrian in the second century A.D., it was completed in 139 A.D after Hadrian had already died (his body was eventually entombed here). Within a hundred years the building was transformed into a fortress to help protect Rome from Germanic invaders. It got its current name in the sixth century—a time when a plague was devastating Rome—after Pope Gregory the Great had a vision of an angel hovering over the structure, sheathing its sword. The vision was interpreted as heralding the end of the plague, and a statue of Archangel Michael, the rescuing angel, was placed on top of the structure (the present bronze statue dates to 1752). In 1277 the fortress was connected to the Vatican Palace with a covered walkway and became a refuge of choice for successive Popes. It also harbored special prisoners, including the acclaimed goldsmith Benvenuto Cellini, who was accused (apparently falsely) of embezzling pontifical gems. Exhibits in the museum today include weaponry and artifacts related to the building’s long, colorful history.
Source: God Bless You: Wikipedia