RIO DE JANEIRO — When the coronavirus first came to Brazil and a call went out for volunteers to work the critical care wards, Isabella Rêllo analyzed the risks. She was 28. She lived alone. She didn't have preexisting conditions.

So while older physicians stepped back from the front lines of the coronavirus response, Rêllo stepped up.

Soon Rêllo, a pediatrician, was treating dozens of coronavirus patients. But they weren’t who she’d expected. This patient was only 30 years old. That one was 32. Nearly half the people she was seeing were young, she said, and many were dying. The narrative seared into the global consciousness in the early months of the pandemic — that the virus spared the young and ravaged the elderly — was not what she was watching unfold in Brazil.

The young were at risk. She was at risk.

“One patient was young, apparently healthy,” she said. “He was so sick, with so many complications. I thought, ‘This could be me. He could be my friend.’ The quickness that this kills people, including the young, has been a shock.”

As the coronavirus escalates its assault on the developing world, the victim profile is beginning to change. The young are dying of covid-19, the disease caused by the novel coronavirus, at rates unseen in wealthier countries — a development that further illustrates the unpredictable nature of the disease as it pushes into new cultural and geographic landscapes.

In Brazil, 15 percent of deaths have been people under 50 — a rate more than 10 times greater than in Italy or Spain. In Mexico, the trend is even more stark: Nearly one-fourth of the dead have been between 25 and 49. In India, officials reported this month that nearly half of the dead were younger than 60. In Rio de Janeiro state, more than two-thirds of hospitalizations are for people younger than 49.

“This is new terrain compared to what’s happened in other countries,” said Daniel Soranz, the former municipal health minister in Rio de Janeiro. “Brazil is a very important country to be looking at.”

Analysts say the emerging data suggests many of the problems that have long troubled the developing world — intractable poverty, extreme inequality, fragile health systems — are increasing vulnerability to the disease. In countries with more poverty and fewer resources, people who might have survived elsewhere are instead dying.

George Gray Molina, chief economist for the United Nations Development Program, said poverty is triggering “compounding effects.” Because population density is so much higher in much of the developing world — and because so many people must keep working to survive — a far greater share of the population ends up being exposed to the virus.

The virus then spreads through a population that’s less resilient. People in the developing world grapple not only with the diseases that have long been associated with it — malaria, dengue, tuberculosis, HIV/AIDS — but increasingly with those more closely associated with wealthier countries. Rates of diabetes, obesity and hypertension are surging. But treatment for many such illnesses is lacking.

When newly infected coronavirus patients already weakened by preexisting conditions seek treatment, they find hospital systems that are overwhelmed and unequipped to handle the deluge of patients.

“It all points to social economic status and poverty,” Gray Molina said. The positive benefits associated with the developing world, such as younger populations, are being “wiped out.”

“As this plays out,” he said, “we will see a balancing of the scales.”

‘My house is empty’

When the coronavirus hit Brazil, it was an infection of the rich. Brought in by travelers to the United States and Europe, the coronavirus circulated primarily among the wealthy and connected. The Brazilian senate leader caught it. So did President Jair Bolsonaro’s press secretary. The Rio de Janeiro Country Club along Ipanema beach, one of Brazil’s most exclusive clubs, suffered a devastating outbreak.

Domingos Alves, a data scientist with the University of São Paulo, has been tracking the virus here since those early weeks. The pattern in Brazil at first mirrored that in the developed world: The dead were almost exclusively elderly. Coronavirus patients were flocking to private hospitals, and anyone who needed a hospital bed received one.

But by early April, as the virus began seeping into the favelas and slums of São Paulo and Rio, and the public hospital system started buckling, Alves noticed a sharp shift in the data. Younger people were being hospitalized at higher rates. People younger than 49 were dying. The disease was reaching lower into the demographic pyramid. The victim profile was changing.

“Our country is made up of various smaller countries,” Alves said. “When you walk through Rio de Janeiro, you go through places that have the characteristics of Switzerland to places more like the Congo, all in the same city.”

Cátia Simone de Lima Passos, 48, has lived her entire life in a part of the city no one would confuse for Switzerland. Every day, she and her daughter, Agatha, 25, would ride crowded buses through northern Rio to the medical clinic where they worked in the favela of Maré. Lima said they did everything they could to stay safe. They doused their hands in sanitizer. They wore masks. Her asthmatic daughter stayed home from work for weeks.

But they both got the coronavirus and were hospitalized. Lima, after 10 days in the hospital, survived. Her daughter didn’t. Now Lima spends her days isolated in her house, alone and unable to grieve with loved ones, trying to understand why a virus that everyone said would kill only the elderly had taken her daughter but spared her.

The unexpected cruelty of it, she said. It’s more than she can bear.

“My house is empty,” she said. “We were partners in life.”

Confusion in the population

Bolsonaro, a global leader in minimizing the virus, repeats a mantra: Only the elderly are at risk. So the best policy is to isolate only them. He has called it “vertical isolation.”

“What has happened in the world has shown that the people at risk are older than 60,” he declared in a national address in late March. “So why close the schools?”

The contradictory messaging in Brazil — between local leaders begging people to stay inside and a president calling people to return to the streets — has fueled widespread confusion. As the virus explodes here, cresting 300,000 cases and 19,000 dead, people are increasingly ignoring isolation guidelines. The beach boardwalks in Rio de Janeiro are packed on weekends. The typical infected person infects nearly three others, according to researchers at Imperial College London, one of the world’s highest rates.

Pedro Archer, a physician at a public hospital in Rio, said his young patients have been stunned by their illness. Some had parroted Bolsonaro, who has repeatedly belittled the illness as a “gripezinha” — a little flu. Until they got sick.

“I have people say to me, ‘I really had thought this was only a gripezinha, and now I see this is serious,’ ” Archer said. “I’ve seen people dying who have said the same thing.”

Others keep going out because they must. Government aid — around $105 per month for informal workers — has for many been either blocked by bureaucratic hurdles or woefully insufficient. Buses are still filled with people heading to work. Lines of people waiting for emergency funds have snaked around banks.

“Young people are dying at a higher rate because they are coming into contact with the virus many times more, because of their working and living conditions,” said Ligia Bahia, a public health professor at the Federal University of Rio de Janeiro. “Doormen are still working. Housekeepers are still working. . . . Their viral load, their exposure, is greater.”

Marcelo Mitidieri, a 48-year-old father of two, understood the risks but continued working as a driver to support his family. He fell sick in late April. He could scarcely breathe. He had pain in his chest. His daughter took him to a medical clinic in the impoverished Rio neighborhood of Engenho de Dentro, but it had only three respirators and three hospital beds. They had no room for him. So he sat in a broken chair for 24 hours, wheezing, texting his daughter Marcela and waiting.

“They want to bring me into the emergency room,” he wrote to his daughter. “But there is no equipment.”

“Try to be calm,” pleaded Marcela, hopeful his age would save him. “Inhale and exhale. You are strong, and we are together on this.”

“I’m very ill,” he responded in his last message before his death.

Marcela now seethes. “If he’d gotten better treatment, he would be with me now,” she said.

All of it has left Rêllo, the 28-year-old pediatrician who volunteered to treat coronavirus patients, terrified. But she kept working — until earlier this week, when she started to feel ill.

A dry cough. Sneezing. Body aches. A test soon confirmed her fears: She’d caught the virus. She doesn’t know what it will do to her. She’s young, but she says she no longer believes that’s enough.

She says she thinks of others whom she treated. She knows what they looked like.

“Like me,” she said.

Marina Lopes in Watercolor, Fla., contributed to this report.