Every day, limbs fall from otherwise healthy-looking trees without provocation there is no tornado, high wind or heavy snow or ice at work.
Sudden limb drops, or "tree failures, occur when a tree develops an internal disease, one that is virtually invisible to the untrained observer and sometimes even tough for the experts to detect.
"This stuff happens, said John McCabe, training and safety coordinator for the Forest Preserve District of Cook County. "It happens on bright sunny days when there is no wind.
Dead trees are obvious to the casual observer, but a tree that is diseased, and therefore fragile, often is discovered only upon close inspection.
Saturday's tragedy, when a tree limb fell on a couple of teens playing flashlight tag in a Will County forest preserve, is a rare event, arborists say, but not unheard of.
In 2002, an 11-year-old Boy Scout from Elk Grove Village was killed when an 80-foot pine tree fell on his tent as a storm approached during a Wisconsin campout.
In June, a baby girl was killed and her mother critically injured when a tree branch snapped and fell about 30 feet, just outside the Central Park Zoo in Manhattan as they were posing for a photo, police said.
Also in Central Park, a computer engineer was seriously injured July 30 when he was hit by a rotting branch off a pin oak tree that fell 20 feet.
Extreme weather is behind most tree failures, but disease can weaken a tree to the point where its limbs drop off, or occasionally the whole tree falls over without warning.
In Chicago, the majority of sudden limb drops have been the result of the emerald ash borer infestation. A significant number of the city's 90,000 ash trees have been infested by the bug since 2009.
Other trees, including oaks and birches, are also prone to sudden limb drops due to infection or disease.
"Oaks can really hide internal decay well, said Andrew Koeser, science and research manger for the International Society of Aboriculture.
Koeser said decaying trees can be identified by closely examining a tree, yet some trees can look healthy on the outside and be completely decayed from fungi, Dutch elm disease or ash borers on the inside.
Another disease trees can be infected with is slime flux, commonly known as wetwood disease. Wetwood can be more easily identified, however, as slime seeps from inside the tree and hardens to form a white crusty substance on the tree's exterior.
Experts are alerted to dead or decayed trees usually when someone notifies them or by performing windshield surveys inspections while driving in vehicles.
Looking for signs of disease, arborists will occasionally bore into a tree to determine if it is hollow a clear indication of a dead or diseased tree.
But with limited resources and hundreds of thousands of trees, the task is challenging, said Malcolm Whitehead, deputy commissioner for the Bureau of Forestry in Chicago's Department of Streets and Sanitation.
"It is really difficult to determine what is going on in the inside, Whitehead said. With over 520,000 trees in Chicago, Whitehead added, "we don't have the resources to bore every tree.
As well, storm-damaged and dead trees get priority over healthy-looking trees.
At the Morton Arboretum in Lisle, which has more than 4,000 types of trees and 14 miles of trail frequented by visitors, arborists regularly look for signs of dead trees or branches to protect the general public from harm, said Doris Taylor, plant clinic manager.
Still, extreme weather is what brings most sick trees down.
"The vast majority of tree failures occur during storms, said Koeser.
"It's fairly unusual for a major branch to collapse (without provocation), added Christopher Pollack, an independent arborist from Naperville.
According to a 2009 study by Kent State University professor Thomas Schmidlin, fallen trees accounted for 407 deaths between 1995 and 2007.
Schmidlin, who has studied tree failures since 2005, found 44 percent of those deaths occurred inside a vehicle; 38 percent were outdoors.