Almost immediately after the deadliest mass shooting in modern U.S. history, which killed at least 59 and injured more than 500 country music festivalgoers, the Sunrise Hospital and Medical Center in Las Vegas realized that it needed help.

It requested physicians from other states to come and treat the victims suffering gunshot wounds and injuries from being trampled while trying to escape the venue where bullets were firing at rapid speed from a room in a high-rise hotel. On Monday evening, Gov. Brian Sandoval issued a public health and medical state of emergency, which will make it easier for out-of-state doctors to get to work.

The emergency declaration reflects not just the large scale of this horrific event but also the state's doctor shortage, which puts Nevada in a uniquely vulnerable position when responding to a mass shooting.

The state routinely has one of the lowest resident-to-physician ratios in the country. A 2015 white paper from Merritt Hawkins, a physician research and consulting firm, ranked Nevada 49 out of 50 states, with only 180 doctors per 100,000 people.

“We need all the medical personnel that are available,” said Amy Shogren, director of communications for the Nevada Hospital Association, which worked with the governor to issue the state of emergency.

A hospital can request emergency services from out-of-state doctors without a statewide emergency declaration, but having the support of the governor’s office helps. Shogren said she doesn't know how many out-of-state doctors to expect.

Whenever doctors come in from other states in an emergency, they must prove they are a licensed physician in good standing to both the state’s health department and a high-ranking hospital member. It’s a logistical hoop, but one that can be done quickly in times of crisis, says Janis Orlowski, chief health care officer for the Association of American Medical Colleges (AAMC). It’s necessary because hospitals can’t just have anyone walking in claiming to be a surgeon, she says.

Requests for out-of-state doctors during an emergency usually correlate with whether or not a state already has a shortage of physicians. After Hurricane Harvey, medical officials in Texas -- which ranks 42 out of 50 in the Merritt Hawkins white paper -- requested out-of-state help. But after the 2013 mass shooting in Washington, D.C, at the Navy Yard, there was no need to bring in doctors from Virginia or Maryland because there were adequate trauma centers nearby, according to Orlowski.

As the country continues to deal with mass shootings and environmental disasters, it’s important that states continue to think about the number of physicians as a key part of their infrastructure, she says.

For its part, the state of Nevada has been trying to address its shortage. It opened a medical school this year at the University of Nevada, Las Vegas, and two Nevadans introduced a bill in Congress that would require more federal funding for medical residencies.

Sixty-eight percent of medical students stay where they trained and did their residency. In 2014, there were 340 residency students in Nevada. The number of residencies varies by state, but across the country, growth is largely flat, according to the AAMC.

*Correction: This story was updated to clarify that 68 percent of medical students stay where they trained and did their residency -- not just where they did their residency.