Three reasons people say bless you when someone sneezes

1) Sneezing causes your heart to stop momentarily. In the middle ages people said "bless you" because they believed it would help their hearts to start again

2) In the middle ages, it was believed that when you sneeze, it was the devil briefly inhabiting your body. Saying "G-d bless you" was a way to insure that the devil would leave your body in a timely manner

3) When sneezing, people often inhale too much oxygen and hallucinate. To scare off the demons that people believed their friends were seeing when they sneezed, people would say "G-d bless you" during the middle ages...similar to reason #2