When someone sneezes, English speakers almost always say "Bless you."
There may be some places lacking this custom, but it is the norm in America, England, Canada, and many other countries. In fact, if you don't say it, it is usually considered rude. Yes, ignoring the person (or worse- simply laughing at them) is very rude. Almost every native English speaker I know says "Bless you," or "God bless you," after a sneeze. Sometimes we even say it to nearby strangers on a bus or in a shop.
Not only English speakers, but people all over the world have some typical response to a sneeze (usually a reference to health or long life). According to Wikipedia, Japan is the only country where, "nothing is said."
So...next time you're speaking English and someone sneezes- you might want to say "Bless you!"
PS: Some English speakers also use the German "Gesundheit." That's good too- it means health.
(article: http://en.wikipedia.org/wiki/Responses_to_sneezing)
