Medical and emergency personnel refer to events like Boston's as a "mass-casualty incident." People who are trained for it have to be ready.
"The first thing that would happen is an activation of a mass-casualty incident, usually by the emergency medical services response team," said Dr. Lee Weiss, the chairman of the Department of Emergency Medicine at Hollywood Presbyterian Medical Center in Los Angeles.
Dr. Weiss and his team have practiced it many times.
"Every day is a disaster in an emergency department, so emergency departments are used to dealing with disasters. But those numbers can be overwhelming," said Dr. Weiss.
When dozens of people are injured, first-responders have to make quick medical assessments. They assign triage tags to casualties.
"Obviously black: deceased; red: most critical and urgent; green: the walking well; yellow is somewhere in between," said Dr. Weiss.
While that takes place, all the personnel in the emergency room assume the roles they've been trained for.
Locally, a system called Ready-Net tells everyone what beds and operating rooms are available.
"Stations have access to that 24-7 and can give critical direction to the ambulances and the paramedics as to where to take their patients," said Dr. Weiss.
In a mass-casualty incident, a disaster cart is rolled out containing equipment for communications and logistics. Because in an emergency situation, not only does the hospital have to take care of its patients, it has to take care of itself.
"If your family is injured and all the hospitals are overwhelmed, trust me: There'll be gang warfare that occurs, and hospitals have to be prepared not only to protect themselves, but to protect everybody that's in their charge," said Dr. Weiss.
While Monday's explosion occurred across the country, all emergency personnel feel close to what's happening.
"Our hearts and our prayers go out to the families and the participants and the injured at this junction, because it's the only thing we can offer," said Dr. Weiss.
When a mass-casualty incident occurs, every hospital will set up its own command center to handle everything from directing patient care to answering family questions.