KATHMANDU, Nepal — His boss fell sick first, with aching pains and a high temperature. Then, some of his colleagues began showing similar symptoms. By the time Rajkumar Adhikari, 35, grew feverish and started vomiting, he learned that the sickness had a name: dengue.

Adhikari, a technician at a company that makes batteries in Nepal’s capital, said he had not heard of dengue before this year, when it struck half of the staff at his office. He felt weak, his nose began to bleed and his joints were so painful it was “hard to bear,” he said from his hospital bed last month.

Nepal, a Himalayan nation wedged between India and China, is experiencing an unprecedented outbreak of dengue this year, part of a broader resurgence in the mosquito-borne viral disease that experts say is linked to a changing climate. In Nepal, there have been nearly 11,000 cases of dengue since mid-July, according to government figures, more than five times the previous annual peak.

Nepal is far from alone. From Brazil to BangladeshHonduras to the Philippines, the number of reported dengue cases has surged this year. There has been a “huge increase,” said Raman Velayudhan, the task force lead for dengue at the World Health Organization. “Unfortunately, things are a little grim at the moment.”

Preliminary figures from the WHO show 2.7 million reported cases worldwide through August. This year’s final tally is likely to match the worst years on record for dengue, Velayudhan said. In the Philippines alone, more than 1,000 people have died. 

Dengue turns out to be ideally suited for a warming, urbanizing world. Present in fewer than 10 countries in the 1970s, it has since spread to more than 120. Dengue is fatal in less than 1 percent of cases and most people recover on their own, but careful supervision is sometimes required to treat severe manifestations of the disease. There is no medication to cure it, nor is there a widely approved vaccine.

As climate change brings rising temperatures and unpredictable rainfall patterns, researchers say that 500 million more people could be at risk of contracting diseases like dengue over the next 30 years. Dengue is “headed out of the tropics and it’s also headed to higher elevations,” said Colin Carlson, a postdoctoral fellow at Georgetown University who studies the link between climate and mosquito-borne illnesses. “This is one of the ways in which climate change threatens global health.”

The mosquito that spreads dengue — Aedes aegypti — thrives in warm temperatures and urban environments. The ideal temperature range for the mosquito to transmit the virus is between about 70 and 93 degrees, researchers say.

Meanwhile, its larvae can breed in the amount of standing water held by a bottle cap. 

“In general, if you have warmer and wetter conditions, it’s more favorable to the mosquito,” said Rachel Lowe, a professor at the London School of Hygiene and Tropical Medicine. But dengue outbreaks can also occur during droughts, when people are forced to store water, she said.

Lowe added that the spread of dengue is also driven by the unplanned growth of cities, whose dense populations provide ample opportunities for contact between humans and mosquitoes and offer plenty of containers where larvae can grow. 

Another worrying characteristic of the dengue virus is its four strains. If people are infected with one strain, they will develop an immunity to it. But if the same people are subsequently infected by a different strain, they are more likely to develop a severe case of dengue, raising the possibility of more serious outbreaks in the future.

Adhikari, the technician in Nepal, was sick for five days before being admitted to the Sukraraj Tropical and Infectious Disease Hospital in Kathmandu. When his colleagues realized that dengue was working its way through the office, they began to kill any mosquitoes they could find and sought out areas where the insects could breed: an upturned lid, a blocked drain, a collection of plastic waste. 

Anup Bastola, a senior doctor at the hospital where Adhikari was treated, said that he saw about a dozen dengue cases last year after the country’s monsoon. This year, the rains arrived early — and the number of dengue patients spiked. Since mid-July, the hospital has diagnosed at least 2,500 cases and admitted more than 260 dengue patients, he said.

“It was terrible. I’ve never seen such a situation in this hospital,” Bastola said. He now spends 11-hour days at the hospital, and his family jokes that to see him, they need to turn on the television, where he has appeared repeatedly to educate the public on dengue and the need for mosquito control. Eight of the hospital’s staff members were infected, and Bastola found a vessel wriggling with mosquito larvae on a hospital balcony.

In Nepal, people were either unaware of dengue, Bastola said, or considered it a disease of the country’s Terai region, the lowlands in the south that border India. But this year, cases have been reported in 67 of the country’s 77 districts, including hilly and mountainous areas. At least six people have died. The country’s agriculture minister and a prominent ophthalmologist were reportedly infected, spurring a minor panic in Kathmandu. 

Bibek Kumar Lal, a doctor who heads the epidemiology and disease-control division of Nepal’s Health Ministry, remarked, half in jest, that his division should be renamed the “dengue control” division. 

In September, Lal said, the prime minister’s office organized a meeting to help coordinate the response to the upsurge in dengue cases. Lal acknowledged that this year’s outbreak was the start of a much longer battle. “Our next generation is going to live with dengue,” he said.

In a bare ward on the second floor of the hospital, several dengue patients rested on their beds as watery sunlight came in the window. Bhupal Dangi, 46, had arrived earlier that morning after driving 11 hours from his home in a district more than 250 miles away. He had been sick for six days, with a bad fever and pain in his joints. He felt so weak he couldn’t clench his fists. 

“The doctors have told us there’s no medicine for dengue,” said his wife, Bhabhi, as she sat on the edge of his bed. “We don’t know what will work.”

Adhikari was in the adjacent bed. The owner of his company, the first to get sick, subsequently recovered. So did his colleagues. But Adhikari’s case turned out to be more serious. “I wasn’t scared at the beginning,” he said. “Then I heard about what could happen the second time I get it. That scared me a lot.”

Ankit Adhikari contributed to this report.