(CNN) "The question is not if something is going to come," said Dr. Mayur Narayan , a trauma surgeon who treated victims of the truck attack in New York last week. "The question is when."

"We're seeing it all around us. We're seeing it in Vegas. We're seeing it in Texas. These things are now unfortunately part of our reality -- our sad reality," added Narayan, an attending surgeon at Weill Cornell Medical Center in the Division of Trauma, Burns, Critical and Acute Care Surgery.

He remembers a time when mass casualty events were a "one-off."

Now, incidents like the shooting in Sutherland Springs, Texas, this week are putting hospitals, first responders and everyday people on high alert. About 20 victims of the Texas shooting were transported to at least three hospitals in and around San Antonio, Freeman Martin of the Texas Department of Public Safety said Sunday.

New York police responded after multiple people were hit by a truck in lower Manhattan in October.

"We are now in an era when you can't just go about your business without being aware of your surroundings," Narayan said. "This is now forcing us as trauma centers to really start to think about, what is our active shooter plan? What is our mass casualty plan?"

Two of the five deadliest mass shootings in modern American history happened this year, within 35 days of each other. Of the 30 deadliest shootings since 1949, 18 have occurred in the past decade.

Surgeons like Narayan are no strangers to the chaos of the trauma bay: the uncontrolled bleeding, the patients getting chest compressions as they're being rolled in. But for some, it's the sheer amount of carnage that's unprecedented.

"We deal with blood a lot, but we don't deal with this magnitude of blood loss all at once," said Dr. Scott Scherr , medical director for the emergency department at Sunrise Hospital and Medical Center, which accepted more than 200 patients last month after a gunman fired repeatedly into a concert crowd in Las Vegas.

In a busy week, the Sunrise emergency room might see six or seven shooting victims, Scherr previously told CNN. But that night, the ER was flooded with hundreds.

Doctors and nurses color-coded the deluge of patients: red for dying. Those patients were sent to trauma bays or operating rooms. Yellow meant life-threatening injuries, but the patient had an hour or so to live. They could wait. Green was for walking wounded.

One of Scherr's colleagues, a doctor who had served in Iraq and Afghanistan, told him it was like "something you would see in a war zone."

Element of surprise

Describing the hospital as a "war zone," whether intentional or not, says something about how trauma centers have been trained to respond in these situations, as experts have pointed out.

"We have unfortunately learned many lessons from the wars in Afghanistan and Iraq," Narayan said. "We learn from our soldiers' experience."

The military has influenced much of how civilian doctors respond to trauma events, such as transporting victims, controlling hemorrhage and dealing with blood transfusions. Narayan, who has trained medics from various branches of the military, said the information they've brought back has saved lives.

"We're giving them skills that will save lives on battlefield. At the same time, the battlefield is teaching us."

But there's one key difference, he added: "In the military, there's an anticipation." In civilian mass casualties, there's "the element of surprise."

Narayan, however, wasn't new to the sudden influx of patients last week, when one man hit multiple bystanders with a rented truck in lower Manhattan. In 2015, he was working at the Shock Trauma Center in Baltimore when riots broke out over the death of Freddie Gray. "I drew on that experience" to effectively mobilize the team in New York, he said.

But many in the hospital hadn't seen anything like it. Narayan and his colleagues tried to build teams in which some members had experience handling a mass casualty incident. They mobilized dozens and dozens of people, from nurses and administrators to janitors, who had minutes to clean the trauma bays before the next set of patients arrived.

But it's not just hospital staff who have suddenly found themselves in the midst of a mass casualty response, he said. Everyday people have been thrust into action before first responders arrive on the scene.

"It's now no longer run, hide, fight -- it's run, hide, fight, help," said Dr. Alexander Eastman , a trauma surgeon and police lieutenant in Dallas.

Eastman, the medical director in chief at the Rees-Jones Trauma Center at Parkland, added, "If you looked at Vegas, very few of those people were initially treated by traditional medical providers, because bystanders ... are on the scene taking care of people."

Concertgoers help an injured person at the scene of a mass shooting in Las Vegas.

Eastman was one of the original experts who advised the White House in creating what became the Stop the Bleed campaign, an effort launched in 2015 to engage bystanders in preventing death from blood loss.

"We've got to give (people) the training and the equipment they need to do that well."

The debrief

"This is a fraternity no one wants to be a part of," Eastman said, echoing a similar comment from Orlando's chief medical examiner, Dr. Joshua Stephany, who was at the scene of a 2016 mass shooting at the Pulse nightclub hours after a gunman killed dozens of people .

Eastman responded to two major active shooters in two years: a 2015 attack on the Dallas Police Department, where he's lead medical officer, and a sniper attack on police officers at a protest last year.

Police barricaded the area after a 2016 sniper shooting in Dallas that left multiple officers dead.

After the second shooting, he received a call from Our Lady of the Lake hospital in Baton Rouge when there was another shooting in that city. They asked him to say a few words at their hospital debriefing and pray with them.

"There's a growing fraternity of us who have dealt with these who find support in talking to each other about what we went through," he said. "Even to this day, 15 months later."

Narayan said that "when your adrenaline's running," the trauma team's training kicks in, and they come together for patients. But they sometimes overlook the psychological impact on themselves, long after their patients have been stabilized and discharged. The moments that stick with Narayan aren't just the tense moments from the operating room but rather the moments when he had to tell family members about their loved ones.

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"There are cases that we are just not going to be able to save," he said.

Speaking as both an officer and a doctor, Eastman said, "we train our whole lives to care for other people.

"When I think about the toll that it takes on you, it's real."