Like Mary Katherine Ladany, the 23-year-old math teacher who was killed on Wednesday by a falling branch, Robert Park was running down an idyllic wooded path on a summer day when he was struck by a tree.

The freak accident in 2000 nearly killed Park, a physicist and writer who lives in College Park, Md. A pair of priests who happened upon him lying unconscious under the tree administered last rites, he later found out.

Insanely unlikely things can happen to us, but Park, who is savvy about risks, doesn't think worrying about them helps.

"As a physicist, my big complaint was that people don't consider the odds and [continue to] worry about things that are terribly unlikely," he said. "I never worried about things that were unlikely, and it came back to bite me."

Officials say Ladany's death on Forbidden Drive was apparently a freak incident, not the result of any avoidable hazard.

"We did a more thorough investigation and found it was a healthy tree," said Mike DiBernardinis, commissioner of Parks and Recreation. Experts say a number of factors, from the weather to heavy vine cover, might have led a healthy-looking 90-foot tulip poplar to lose a massive branch. But all agree that such events are rare.

DiBernardinis said the park commission tries to watch over its estimated 2.5 million trees, looking for uprooted, diseased, and aging ones that pose a danger, "but in this instance the daily routines along the trail didn't pick it up." He said park arborists would conduct a sight survey as soon as today to look for diseased trees.

About 700 people a year die from falling or thrown objects, said Robert Anderson, chief of mortality statistics at the Centers for Disease Control and Prevention. That number includes rocks, debris, and manmade objects that fall or are thrown. A much smaller subset of victims are felled by trees or branches.

Experts agreed such a thing can happen even in a well-maintained park.

"Trees lose limbs randomly for all kinds of reasons," said Timothy Dugan, a forester with the Department of Conservation and Natural Resources. "Trees have a strong ability to cover up problems internally."

They can even keep producing leaves when they're dying inside, he said.

Several factors might make trees more precarious now. Many are supporting woody vines called lianas, said Alfred Schuyler, a professor of botany at the Academy of Natural Sciences. Those vines, especially English ivy and Oriental bittersweet, aren't native to this area and can weaken trees.

The weather poses another danger, said Eva Monheim, a certified arborist and professor of environmental design at Temple University. This year has been the first to give us normal rainfall after a series of dry years, she said. Dry years can weaken the root systems of trees though they may appear alive and healthy above ground. Add saturating rains and strong winds, and an apparently healthy tree could fall over, she said.

She said all the foot traffic through city parks also weakens trees by compressing the soil and essentially squeezing out the air. Still, she saw no problem with park maintenance.

Other things are far more dangerous, the CDC's Anderson said.

Car crashes, for example. They kill about 40,000 people every year.

People tend to overestimate small risks thanks to the fallacy of the small sample, said risk consultant David Ropeik, author of the book How Risky is it, Really? to be published by McGraw-Hill in March.

Even though a man was injured by a tree in Central Park on July 29, for example, that does not mean the risk is increasing. "It's the same thing with cancer clusters and people freaking out because a study associated product X with disease Y," he said.

We also tend to worry more about risks when we think we don't have control, he said. People who write text messages while driving pose an obvious risk to themselves and others, but they delude themselves into thinking they're in control.

Park, the 2000 tree-fall victim, understands that unlikely events will happen on occasion, such as a strange coincidence that took place the day he returned to the scene of his accident a year later.

"The story gets even more unbelievable," he said. He went to the exact place where he was struck, he said, and as he passed the broken-off trunk of the tree that nearly killed him, he passed two elderly men walking. "You know that tree fell on a guy last year," one of them said.

When Park said he was that man, one of the two began to tear up. It turned out they were the priests who found Park pinned under the tree and gave him last rites. They decided to throw him a champagne party to celebrate his survival.

Park said that in his case there was a dangerous situation: the tree had been obviously dead. He's now on a commission to help keep the trail safer but still considers his accident a freak act. "You can't live your life worrying about improbable events or you'll never get out of your house."

Contact staff writer Faye Flam at 215-854-4977 or fflam@phillynews.com.