Why do people say "bless you" after someone sneezes?

Throughout history, many cultures believed that a sneeze was the beginning of an illness, and they would wish blessings or good health on the person who sneezed. According to HowStuffWorks, a similar phrase is Germany is "gesundheit."

During Pope Gregory's reign in the sixth century, sneezing heralded the onset of the Bubonic Plague, and it became a widespread practice to invoke blessings upon any who sneezed in order to entreat protection from God. During the Middle Ages, it was believed that the heart stopped for the short moment during a sneeze, and the soul left the body. Blessing the person who sneezed was a way of welcoming the soul back to the body.