I've heard that the custom of saying tzu gezunt, bless you etc. When someone sneezes is because until Ya'akov, people used to sneeze and die, but Ya'akov was the first one to first get sick. What is the source for this.
The Pirkei D'Reb Eliezer 52(starting from the words Mofes Arba) writes that from creation until Yaakov people would go out to the shuk and they would sneeze and then die without sickness beforehand.
Until Yaakov there was no sickness, once Yaakov came he begged for mercy and there was sickness. Bava Metzia 87a