But this adaptation comes at a price -- and it can cost some people their lives.
"It's a major stress for the organs," Samuels said. "A certain number of people actually have abnormal heart rhythms as a result of these chemicals. A person will actually drop dead on the spot."
Fortunately, Hammill said, it is exceedingly rare for a person to be scared to death.
"Obviously, many people are frightened each day and do not die," he said.
Why some people die from fright while most of us do not isn't completely known, Samuels noted. But once this question is answered, the knowledge may lead to ways to prevent such deaths from occurring.
"One would predict that there might be a way to block this and prevent this damage from happening -- not just in the heart, but in other organs as well," Samuels said. "These deaths are tragic, but a great deal is learned from these types of events."
Associated Press reports contributed to this story.