My usual answer is "Thanks."
You know something, Ana, I have no idea why people say "God bless you", or just "Bless you," after a sneeze! So I had to look it up:
"In English-speaking countries, it is common for at least one person to say "Bless you" after someone sneezes. The traditional explanation for the custom is that it began literally as a blessing. Pope Gregory I the Great (540-604 AD) ascended to the Papacy just in time for the start of the plague in 590 AD (his successor succumbed to it). To combat the plague Gregory ordered litanies, processions and unceasing prayer for God's intercession. When someone sneezed, they were immediately blessed ("God bless you!") in the hope that they would not subsequently develop the plague.http://en.wikipedia.org/wiki/Sneeze
Some believe it is because when someone sneezes his heart stops and saying "God bless you" means "I'm glad your heart started again."
According to Millhouse on the Simpsons: When you sneeze your soul tries to escape and saying "God bless you" crams it back in.
During the sixth century, it was customary to congratulate people who sneezed because they were expelling evil from their bodies. Later, when a great plague took hold of Europe, and people began sneezing violently, the Pope passed a law. Since sneezing meant that the person was going to die of plague, people were required to bless the sneezer."
Some people say "Gesundheit" instead of "Bless you." Has anyone heard that one?