In the old days, people thought that their souls left their bodies when they sneezed. They were worried that their souls might not get back into their bodies safely. So they said "Bless you" to pray for the safe return of the soul. No one believes it today, but people still say "Bless you" when they see somebody sneeze.

(What does 'it' refer to in 'believe it today'?)

Didn't I already answer this a few days ago?

it = the belief, the suoerstition

Thank you. Then does 'it' refer to all the sentences about the belief?

Yes.

In the sentence, "No one believes it today," the word "it" refers to the belief or idea that the souls of people leave their bodies when they sneeze. The sentence is stating that nowadays, most people do not hold this belief anymore.