It happens everywhere -- on the street, at the mall, in the movies. During hay fever season, it happens even more.
Sneeze and people around you -- perfect strangers! -- feel a compulsion to say, "God bless you," or simply, "Bless you."
If you've been schooled in polite behavior, you'll respond, "Thank you."
You can't escape it. Salutation on sternutation -- the act of sneezing -- is universal.
"Gesundheit!" say Germans.
"Yarhamak Allah," say Arabs.
"Tihei mauri ora," say Polynesians.
But where did this bit of social etiquette originate?
It came from a long way back, from the time of the ancient Romans and Greeks -- and even earlier.
"It comes from the idea that you are sneezing out your soul," said Moira Smith, librarian at the Folklore Institute at Indiana University in Bloomington.
In primitive belief, the soul, which animated every living thing, was flighty, she said. You could be alive one minute and dead the next if you weren't careful about the soul flying off.
"There are a lot of ancient beliefs in general about a separable soul. It could separate for a brief period of time. When you dream, your soul is out of your body, so it can't get back in if you're sneezing," she said. Even yawning posed a danger.
Smith pointed out that the idea of invoking divine protection in a sneeze was documented as early as the 1st century by the Roman naturalist Pliny the Elder.
In 77 A.D., Pliny wrote in his "Natural History": "Why is it that we salute a person when he sneezes, an observation which Tiberius Caesar, they say, the most unsociable of men, as we all know, used to exact, when riding in his chariot even?"
The beginning of the practice is lost in the mist of prehistory.
Coupled with the idea of a soul getting away was the fear that an evil spirit could enter the body through the mouth or nostrils and steal away an unguarded soul. That made a blessing doubly necessary.
"The Golden Bough: A Study in Magic and Religion," the monumental 13-volume compendium of superstitions by Sir James George Frazer, devotes an entire section to "the perils of the soul."
A more up-to-date variation on this theme has been the notion that the heart stops when you sneeze -- a potentially fatal state of being. Alas, that's not the case.
"It does not stop whatsoever," said Jose Missri, chief of cardiology and chairman of the Department of Medicine at St. Francis Hospital and Medical Center in Hartford, Conn. He said he, too, has been curious about why people say "Bless you" when someone sneezes.
Robert V. Blystone, who teaches biology at Trinity University in San Antonio, sees a common thread running through this ancient concern about sneezing.
"In a world without antibiotics, a sneeze could mean the start of an illness which could kill," he said. There were "no magic bullets to take care of the aftereffects of the early warning system called a sneeze."
Many people today have no appreciation for what it was like to live without the miracles of modern medicine, he said.
Blystone said he remembers not being allowed to go swimming because of the danger of polio.
"I can still remember the quarantine signs on houses with kids who had the measles. I remember the fear in the faces of parents when a child was diagnosed with strep throat. Today, the child is worried about missing the afternoon baseball game because of strep throat."
Blystone recently participated in an Internet discussion among biologists on the subject.
"Can anyone help me with this question?" asked Professor Deborah M. Langsam, of the University of North Carolina in Charlotte. "I usually don't have any trouble with the bacteriology' questions I get via e-mail, but this one has me stumped!"
The question, from a biology student, was: "I was just wondering why people say Bless you' when someone sneezes. Is it because your heart stops when you sneeze, or is it because it was once believed that your soul left your body when you sneeze?" Langsam commented: "A lot of cultures seem to have this sneezing thing. . . . I think the student is right that in some cultures it had to do with danger to the soul -- but the concern from what I understand is that sneezing caused the body to be unprotected so that evil spirits/demons could enter."
Her plea prompted a flurry of responses. The consensus was there are no really wrong answers. It all depends on what your grandmother told you.