There is something about a bomb attack - like the letter bombings in the Unabomb case or the fire-bombing on the subway - that provoke a particularly intense fear.

The explosions seem to come out of nowhere, transforming an ordinary routine like going to work or opening a package into a deadly act. And, there is the randomness of it, the fact that it can seemingly happen anyplace and at any time.Yet despite last week's explosion on the subway, which injured 40 people, as well as the vastly larger blast at the World Trade Center nearly two years ago, officials and experts on security and terrorism were cautioning against overreacting to what were terrible but very rare incidents.

"I don't mean to sound callous about the tragedy of people injured in the subway attack, or killed by the Unabomber," said Brian Jenkins, the deputy chairman of Kroll Associates, an investigative and corporate security firm. "But in the United States, with 24,000 homicides a year, an American has a 1-in-10,000 probability of being murdered, while the odds of being killed by a terrorist bomb are something less than 1 in 100 million, which is a ten thousand-fold difference."

The incidents and the resulting consternation highlight a paradox. On one hand, the incidents produce a kind of fear on the part of the public, in large part because they attract inordinate amounts of publicity. But the fact remains that terrorism, while scary and headline-grabbing, is, compared to other hazards, a minor threat.

"There is no fail-safe for these things, just as there are no fail-safes if you're walking on a street or in a shopping mall," said Lt. Robert J. Valentino, a spokesman for the transit police. "But these are very isolated incidents. We can't even think of the last time something like that happened."

What has made the letter-bomb killing of a New Jersey advertising executive and last week's subway bomb seem particularly alarming is that they both occurred while people still have fresh memories of the explosion at the World Trade Center in February 1993, in which six people were killed.

The Trade Center blast was a mammoth one, and the recent explosions minuscule in comparison.