The answer broke her heart.

 "When La Mara broke all my teeth and killed all my family," the 14-year-old said.

He said little else about the attack by the infamous Central American gang, La Mara Salvatrucha. Just: "I was the only one that survived."

Calderon is not a therapist, nor a lawyer or a dentist. She is a general practitioner volunteering to provide care for Central Americans stuck in Mexico while they try to obtain asylum in the United States. There was little she could do for this teenager.

"So I gave him an antibiotic, then went home and cried," she said.

Calderon is part of a movement of health professionals and medical students from both sides of the U.S.-Mexico border that is quietly battling to keep asylum seekers healthy and safe while their lives are in flux.

They try desperately to tend to a need left largely unmet by the governments of both countries. It has thrust volunteer doctors into new and unusual roles where they often have to improvise while working with limited donated medications and equipment and dealing with non-medical issues. Besides giving patients a pill for pain relief, the doctors might need to direct them to legal help for their cases while offering a listening ear as a kind of therapist to a population suffering deep trauma from violence that forced them to flee their homelands.

With little training or preparation for this type of medical work, doctors like Calderon are trying to come up with guidelines to better treat migrants with emotional trauma.

Tens of thousands of people are stuck in Mexican border cities as their asylum cases wind their way through the U.S. court system under a Trump administration policy that returns them across the border to wait out a decision. Thousands of others wait for their numbers to be called so they can start their claim in a process that meters the number of asylum requests that are submitted to U.S. officials.

Along the U.S.-Mexico border, thousands are living in crowded shelters or outside in makeshift tents.

The health crisis spans both sides of the border. In the past year, at least three children, detained by U.S. Border Patrol agents, have died from the flu while being held.

Meanwhile, in Tijuana, volunteers like Calderon have been setting up weekend, pop-up clinics at shelters rarely visited by Mexico's public health doctors.

"I shouldn't be doing this," said the 34-year-old Tijuana doctor whose private practice caters to uninsured Americans. "They need to be in another place to be safe. That other place should be taking care of them, or the Mexican government should be taking care of these refugees."

She helps run the Refugee Health Alliance, one of a handful of such groups along the 1,954-mile border.

In the past year, the Refugee Health Alliance has hosted 800 volunteers who have seen more than 9,000 patients; in addition to treatment, they document signs of torture and abuse for asylum cases.

On the group's 52nd consecutive Saturday at the shelters, the volunteers are given a brief orientation: Don't ask about people's backgrounds, which could trigger traumatic memories, or take photographs. Fill out medical forms that ask for a person's medical history and dates for their asylum process, which could disrupt their care.

The volunteers also are told to put labels on any medication given to the asylum seekers so U.S. immigration officials will not take the pills away — though they often do, anyway.

Their first stop: the bottom of "Scorpion Canyon."

Calderon leads the volunteers into a Christian church that first sheltered Haitians who flocked by the thousands to this border city in 2016. Now the church is filled with scores of tents housing Central Americans.

The two dozen volunteers include two pediatricians, a university professor who also practices medicine, medical students from Phoenix and San Francisco, a Stanford University psychology doctorate student who worked with children at refugee camps in Iran, and two sisters from Los Angeles who have relatives in Tijuana.

They unfold tables and metal chairs in the congregation hall to make makeshift examining spaces. They unpacked a half-dozen duffel bags and suitcases bursting with plastic bags filled with donated medications brought in by volunteers.

Calderon works between the church where she sees mostly Central Americans and a neighboring cluster of rooms housing several dozen Haitians.

She sees a woman with a badly healed broken wrist, a girl's belly covered with scabies, an undernourished pregnant woman, a baby with a cold, a toddler who is underweight, a woman with a swollen check and infected tooth, another with a red, swollen eye.

With a warm smile and a pink stethoscope around her neck, she saw patient after patient.

Eight months have passed since that day she saw the boy with the broken teeth. She still thinks of him. She never saw him again.

The experience made her a better doctor, she said. Now when Calderon asks about their pain, she treads carefully: A hurt neck, might be from getting a head smashed in by thieves. A case of acid reflux might stem from anxiety about not being in a safe place.

"When you see someone who comes to you with insomnia, with no hope, it's really hard on us too. What do we say? What do we treat? Is this an illness?" she asks. "That's why we're trying to write the protocol for mental health, and trying to get experts for refugee medicine to help us out with these questions."

Calderon always wishes she could do more.

But she has learned to accept the limitations after treating the boy with the toothache. She took time off and sought therapy for herself, feeling overwhelmed. Then she took a course to learn how to treat patients who have endured tragedies.

"I need to be OK that I did something," she said. "It's a thing that all doctors come to understand at some point, right? I hope. We do what we can."