Cases of dengue fever, a mosquito-borne viral illness that can be fatal, are surging around the world. The increase is occurring both in places that have long struggled with the disease and in areas where its spread was unheard-of until the last year or two, including France, Italy and Chad, in central Africa. Last week, health officials in Pasadena, Calif., reported a first case of locally transmitted dengue.

What is dengue fever and why is it becoming more widespread?

Dengue, a viral fever, is transmitted by Aedes species of mosquitoes. It can cause excruciating joint pain; is also known by the grim nickname “breakbone fever.”

The Aedes aegypti mosquito, which has been driving many of the current outbreaks, is native to Africa, where it originally lived in forests and fed on animals. But decades ago it spread to the rest of the world via trade routes and adapted to thrive in urban areas, feeding on people and breeding in small bits of trapped water in places such as old tires, discarded bottle caps and trays used to catch air-conditioner drips.

Now as more people move to urban areas — many to lower-quality housing in developing countries — they are more vulnerable to the virus. And climate change is bringing the mosquito to new places, where it is thriving.

“Aedes mosquitoes thrive in warm and humid environments, so definitely climate change and rising temperatures and also extreme weather events are helping extend their habitat range,” said Dr. Gabriela Paz-Bailey, chief of the dengue branch at the division of vector-borne diseases at the U.S. National Center for Emerging and Zoonotic Infectious Diseases.

How dangerous is it?

Only one in four dengue cases are symptomatic. Some infections may produce only a mild flulike illness. But others can result in dreadful symptoms, including headache, vomiting, high fever and the aching joint pain. Full recovery can take weeks.

About 5 percent of people who get sick will progress to what’s called severe dengue, which causes plasma, the protein-rich fluid component of blood, to leak out of blood vessels. Some patients may go into shock, causing organ failure.

Severe dengue has a mortality rate of between 2 and 5 percent in people whose symptoms are treated. When left untreated, however, the mortality rate is 15 percent.

Severe dengue may go untreated because patients live far from medical care or can’t afford it, or because hospitals are overwhelmed with cases during an outbreak.

Or it can happen when dengue isn’t diagnosed in time because it’s appearing in a new area. That is a problem in resource-strapped countries such as Chad, of course, but also a challenge for a patient or clinician in Pasadena who is unfamiliar with dengue, Dr. Paz-Bailey said.

Who is at risk?

Forty percent of people globally live in areas where they are at risk of dengue infection; the disease is most common in tropical countries such as Brazil.

The people most vulnerable to dengue live in housing that doesn’t keep mosquitoes away from them. In studies on communities along the U.S. southern border in areas where the aegypti mosquito is well established, researchers found that there were as many or sometimes even more of the mosquitoes on the Texas side, but far fewer dengue cases there than on the Mexican side.

That is because more people on the U.S. side of the border had screened windows and air-conditioners, which limited their exposure to mosquitoes, and lived farther apart and were less social: By making fewer visits to friends and relatives, they were less likely to take the virus into new areas where a mosquito might pick it up from them and pass it on.

It is unlikely that dengue will become a serious problem in the United States, “as long as people keeping living like they’re living now,” said Thomas W. Scott, a dengue epidemiologist and professor emeritus at University of California, Davis.

Outside Puerto Rico and other territories where the disease is endemic, there are about 550 dengue cases each year in the United States, but they are imported by travelers who were infected abroad and passed the disease along to their close contacts.

The case in Pasadena is a rare locally acquired case of dengue in the United States. City officials said on Friday that they had been trapping and testing mosquitoes in the neighborhood where the case was reported and had not found any further insects with the virus.

But scientists say dengue will continue to spread to places that haven’t experienced it before.

In addition to climate change, rising rates of urbanization around the world are playing a role, said Alex Perkins, who is an associate professor of biological sciences at the University of Notre Dame and an expert in the mathematical modeling of dengue transmission. If people have recently come from rural areas, they are unlikely to have priority immunity, so the virus can move swiftly through the population.

“I don’t think that the case in Pasadena or anything else that I’ve seen lately is an indication of any looming crisis in the United States in the short term,” Dr. Perkins said. “But I think the general expectation that this is going to be a growing problem in the United States is reasonable.”

Dr. Perkins said the experience of southern China offers a cautionary tale: Historically, the region saw only a handful of dengue cases each year. Then in 2014, there were 42,000 cases in Guangdong Province. “All of a sudden in one year, it grew by a couple of orders of magnitude without any real forewarning,” he said.

He added, “In endemic settings, we’re continuing to have record years, year in and year out, and that’s what’s driving all these imported cases in the United States and elsewhere. And when it comes to the more marginal transmission settings such as the southern United States, southern Europe, China — it’s not getting better there either. So it’s really getting better nowhere: it’s all bad.”

Is there treatment for dengue?

There is at present no specific treatment for dengue infection, and patients are given only symptom management, such as medication to control pain. Drug companies have antivirals in clinical trials.

Is there a vaccine?

The effort to find a dengue vaccine has been long and complicated. Dengvaxia, a vaccine developed by the French firm Sanofi, was rolled out widely in countries such as the Philippines and Brazil starting in 2015, but two years later the company said it was causing vaccinated people who caught the virus to have more severe cases. The U.S. Centers for Disease Control and Prevention recommends use of Dengvaxia in endemic areas for people with a laboratory-confirmed previous dengue infection.

The World Health Organization recently recommended a new vaccine called QDENGA, which can be used regardless of prior infection status, for children aged 6 to 16 living in areas with high dengue transmission. This vaccine has already been introduced in Indonesia, Brazil, Thailand and 16 European countries, including Britain and Italy, but it won’t be available in the U.S. any time soon. Takeda, the Japanese company that makes it, withdrew it from the F.D.A. approval process in July after a dispute over what data was being considered.

What else can we do?

Some countries that have acted aggressively against dengue have controlled it. Singapore uses a combination of methods, including the inspection of homes and construction sites for breeding areas, with high fines for rules violations. “It’s a successful approach but they have a really large budget to support those activities, but not every country has that,” Dr. Paz-Bailey said.

Brazil and Colombia have had success using a bacteria called Wolbachia: When Aedes aegypti is infected with the bacteria, it can no longer pass on the dengue virus. Projects there are mass-producing mosquitoes infected with Wolbachia, and releasing them to breed with wild insects in an effort to get the bacteria through the mosquito population.