Her Facebook post has been shared nearly half a million times.
Andrea Pergola wrote that her son Logan was picking up tree branches in the yard at their home in Land O'Lakes, Florida, when something brushed up against his arm. He felt a sharp, stinging sensation and within minutes was dizzy, nauseated and in terrible pain.
She rushed him to the hospital where he became more disoriented, the pain intensified and a rash spread up his arm.
"The pain was radiating from his wrist, up his arm and into his shoulder and chest," Pergola wrote. "The rash also spread up his entire arm and into his chest."
Doctors said the spotted rash represented dozens of stings -- well over 20 injection sites.
Pergola says the caterpillar was from a Southern Flannel Moth. Her son recovered within a few hours, but Pergola is warning people to be aware of how dangerous the critters are.
"He is a healthy, strong, young man and it knocked him out," she wrote. "I can't even imagine a small child or elderly person. Please research this caterpillar, be aware of it and make your kids aware of it."