Brazil is experiencing an enormous outbreak of dengue fever, the sometimes fatal mosquito-borne disease, and public health experts say it is a harbinger of a coming surge in cases in the Americas, including Puerto Rico.

Brazil’s Health Ministry warns that it expects more than 4.2 million cases this year, outstripping the 4.1 million cases the Pan-American Health Organization recorded for all 42 countries in the region last year.

Brazil was due for a bad dengue year — numbers of cases of the virus typically rise and fall on a roughly four-year cycle — but experts say a number of factors, including El Niño and climate change, have significantly amplified the problem this year.

“The record heat in the country and the above-average rainfall since last year, even before the summer, have increased the number of mosquito breeding sites in Brazil, even in regions that had few cases of the disease,” Brazil’s health minister, Nísia Trindade, said.

Dengue case numbers have already soared in Argentina, Uruguay and Paraguay in the last few months, during the Southern Hemisphere summer, and the virus will move up through the continents with the seasons.

“When we see waves in one country, we will generally see waves in other countries, that’s how interconnected we are,” said Dr. Albert Ko, an expert on dengue in Brazil and a professor of public health at Yale University.

The World Health Organization has warned that dengue is rapidly becoming an urgent global health problem, with a record number of cases last year and outbreaks in places, such as France, that have historically never reported the disease.

In the United States, Dr. Gabriela Paz-Bailey, chief of the dengue branch at the division of vector-borne diseases at the Centers for Disease Control and Prevention, said that she expected high rates of dengue infection in Puerto Rico this year and that there would be more cases in the continental United States as well, especially in Florida, as well as in Texas, Arizona and Southern California.

Dengue is spread by Aedes aegypti, a species of mosquito that is becoming established in new regions, including warmer, wetter parts of the United States, where it had never been seen until the past few years.

Cases in the United States are still expected to be relatively few this year — in the hundreds, not millions — because of the prevalence of air-conditioning and window screens. But Dr. Paz-Bailey warned: “When you’re looking at trends in numbers of cases in the Americas, it’s scary. It’s been increasing consistently.”

Florida reported its highest number of locally acquired cases last year, 168, and California reported its first such cases.

Three-quarters of people who are infected with dengue don’t have any symptoms at all, and among those who do, most cases will resemble only a mild flu. But some dengue infections are serious, producing headaches, vomiting, high fever and the aching joint pain that gives the disease the nickname “breakbone fever.” A bad dengue case can leave a person debilitated for weeks.

And about 5 percent of people who become 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 percent and 5 percent in people whose symptoms are treated with blood transfusions and intravenous fluids. When left untreated, however, the mortality rate is 15 percent.

In Brazil, state governments are setting up emergency centers to test people for dengue and treat them. The city of Rio de Janeiro declared a public health emergency over dengue on Monday, days before the start of the annual celebration of Carnaval, which brings tens of thousands of people to outdoor parties through the days and nights.

High numbers of cases are being reported in the southernmost states of Brazil, Ms. Trindade, the health minister, said, which are typically much cooler than Rio and the states in the center and the north. People in those areas will have little immunity to the disease from prior exposure.

Dengue comes in four different serotypes, which are like virus cousins. Previous infection with one offers only short-term protection against infection with another, and a person who has had one serotype of dengue in the past is at higher risk of developing severe dengue from infection with another serotype.

“Right now you have serotypes circulating in Brazil that have not circulated in 20 years,” said Dr. Ernesto Marques, an associate professor of infectious diseases and microbiology at the University of Pittsburgh.

Brazil has started an emergency campaign to immunize children in areas with the highest rates or risk of dengue transmission, using a two-dose vaccine called Qdenga that is made by the Takeda Pharmaceutical Company of Japan. Brazil bought 5.2 million doses for delivery this year, plus nine million more for delivery in 2025, and the company donated an additional 1.3 million, which effectively locks up most of the supply of Qdenga globally. A company spokeswoman said Takeda is working on a plan to increase supply, focusing on delivery to high-prevalence countries.

But even so, that is enough to cover less than 10 percent of the Brazilian population over two years. The only good news about dengue in Brazil at the moment is the publication of clinical trial results for a new vaccine tested by the public health research center Instituto Butantan in São Paulo. That vaccine requires just one shot, and the trial found that it protected 80 percent of those vaccinated against developing dengue virus disease. The research center will ask the Brazilian government to approve the vaccine, and it has facilities to produce it, aiming to start delivering shots in 2025.

For this outbreak, it’s too late for vaccination to help much, and there are few other ways for the public health authorities to slow it down.

“Insecticide resistance really limits what you can do in terms of controlling the mosquito population, and insecticide resistance is widespread,” said Dr. Paz-Bailey at the C.D.C. “What you can do is ensure that people have access to clinical management and that clinicians know what to do.”

Medical centers in Brazil are setting up extra beds for people with severe dengue, hoping to prevent the kind of overwhelming of health systems that occurred during the Covid-19 pandemic and to prevent dengue deaths.

“The old paradigm of dengue most affecting children is not the case in Brazil — you have to think about the elderly, who are very vulnerable,” Dr. Ko said. It will be important for both clinicians and the public to get the message to test for dengue at the first sign of symptoms in both children and older people, he said.

“Any educated guess was that this would be a bad year,” Dr. Marques said, “but now we know how bad. It’s going to be very, very bad.”

Lis Moriconi contributed reporting from Rio de Janeiro.