What is the origin of saying "gezuntheit" or "G‑d bless you" after someone sneezes?
Although not technically part of Jewish Law (halachah), saying gezuntheit, tzu gezunt, labreeyut, or G‑d bless you is considered a mannerly custom. It is written in the Midrash that the Patriarch Jacob was the first person to become ill before passing on. Before that, people would sneeze and die. When G‑d infused the soul into Man, He "blew it" into Adam's nostrils. Thus, when it came time for the soul to be returned to its Maker, it would leave through the same portal it arrived.
Furthermore, sneezing has always been an indication of being sick. During the Middle Ages, when plague was common, and a sneeze could have meant a serious, incurable disease, people would wish that person to be blessed.
My Bubby taught me that the first time a person sneezes it is proper to say tzu gezunt – to health; the second time, tzum leben – to life; and the third time, tzu lange yoren – to long years.
Many have the custom after sneezing to gently tug one's earlobe and recite the verse, "lee-shua-techa kiviti Adonai – I hope for your salvation, O G‑d."