The shooting at the Mercy Fitzgerald Hospital in Darby, PA was stopped by Dr. Lee Silverman, who happened to have his gun with him. Silverman critically wounded the attacker Richard Plotts after Plotts shot to death case worker Theresa Hunt. Hunt was apparently taking Plotts to his appointment when the shooting occurred. Silverman intervened before more people could be harmed and shot Plotts three times.
The Philadelphia Inquirer has a quote from someone at the hospital:
Whether Silverman had a special exemption from the hospital to carry isn’t clear, but the benefit from him doing so convinced the local police chief to note:
“Without that firearm, this guy (the patient) could have went out in the hallway and just walked down the offices until he ran out of ammunition,” the chief said. . . .
It appears extremely likely that this doctor was one on the 870,000 concealed handgun permit holders in Pennsylvania (8.7 percent of the adult population).
A detailed discussion is also available here.