The Philippines, Bangladesh, and Honduras are dealing with major, seemingly uncontrollable outbreaks. Others have popped up in Palau, Nepal, Guam, the Cote D’Ivoire, the Cook Islands, French Polynesia, and in other places throughout the tropics.

There’s no question about it: Dengue is having an active year.

“This is turning out to be a very bad year for dengue, probably the worst we’ve had since 2010,” said Tyler Sharp, an epidemiologist at the CDC’s Dengue Branch in Puerto Rico.

In the Marshall Islands, a tiny nation of about 53,000, Jack Niedenthal has seen about 600 cases since June. “There’s never been a bigger outbreak of dengue in the Pacific. It’s awful. It’s really caused a lot of hardship,” said Niedenthall, the nation’s Secretary of Health and Human Services. A six-year-old girl died of the disease in early September; Niedenthal described it as his “worst day ever” while serving in the role.

Thanks to a travel ban – designed to keep the disease from transmitting out to the country’s less populous islands, many of which lack the medical resources, or even basics like electricity and refrigeration – and an extensive education and containment effort, the Marshall Islands’ outbreak has thus far been contained to Majuro, the nation’s largest city, and Ebeye, an island.

But new cases crop up every day.

“Once you have a dengue outbreak that’s occurring, it’s quite difficult to stop it. It’s also difficult to contain it,” Sharp explained.

It hasn’t historically gotten as much attention as malaria – a mosquito-borne parasite that received widespread international attention beginning in the mid-20th century, resulting in an overall decline – or Zika, a virus that gained widespread attention during the widespread 2015-2016 outbreak, in part because of the disease’s sometimes devastating effect on newborn infants.

But dengue is fundamentally different from malaria in one vital way, said Oliver Brady, an epidemiologist at the London School of Hygiene and Tropical Medicine. “Vector-borne diseases are less prevalent now than they were 10 or 20 years ago, but for dengue that’s not the case,” he said. “It’s one of the few infectious diseases at a global scale that’s still increasing and expanding.”

It’s hard to know how many dengue cases occur each year, although some estimates place the number as high as 390 million. But many people never develop dengue symptoms, or don’t seek treatment unless their symptoms are severe, said Sujan Shresta, an infectious disease researcher at the La Jolla Institute for Immunology.

In addition, she explained, dengue affects a number of poor countries that don’t necessarily have the infrastructure or resources to carefully track the disease. “Record keeping is not up to par,” she said.

For more information on dengue’s spread during this and previous years, visit the interactive map and dashboard below.

An Expanding Mystery

Dengue does tend to work on a cycle, breaking out once every three to five years. After each outbreak, “the good news is that a lot of people have been infected, so you have herd immunity that lasts for awhile, until the next cycle,” Shresta said.

Just to make things more complicated, scientists have hypothesized – although the theory has not yet been proven – that exposure to Zika virus may actually offer temporary protection against dengue for a few years. Researchers noted that the years immediately following the Zika outbreak brought remarkably few dengue cases.

However, “it may actually start enhancing infection” after those few years, Shresta said. In other words, having had Zika may actually make people more vulnerable to dengue after the initial protection wears off, which could help to explain dengue’s prevalence this year.

Dengue is a complicated disease, and scientists don’t fully understand why it interacts with our immune systems the way it does – why, for example, a person who contracts dengue more than once tends to get sicker the second time. Why only 25% of people who contract the disease become symptomatic. Why only a tiny number of cases – about 0.5% – will turn into the severe dengue that puts lives at risk and gives the disease its distinctive nickname: “breakbone fever.”

And in addition, there isn’t a cure or even a reliable vaccine, although a vaccine introduced in the Philippines caused panic when it was found to have caused the deaths of several children. In general, people affected by the disease are carefully monitored, given IV fluids, and sometimes transfused, Sharp said.

A Contested Sphere

But what seems to be clear is that dengue will play a larger and larger role in our lives, although scientists don’t agree on precisely what that role will look like.

General wisdom suggests that as the climate warms, the breeding ground of the Aedes aegypti mosquito – the species responsible for transmitting dengue, Zika, chikungunya, and yellow fever, among other diseases – will spread into previously temperate regions, including in the U.S. and Europe. (The mosquito can already be found in the United States’ warmer climates.)

“Especially for people in North America and Europe, this is happening,” Shresta said. “I see this as a really quintessential global public health problem,” as opposed to one relevant just to tropical or poor countries. “We need to help these countries understand these viruses because that in turn will help us.”

For Brady, lead author of a new dengue modeling study, the picture looks a bit different. Traditional attempts to model the future of dengue’s spread are primarily based on temperature, he said – in other words, they assume that any location that reaches the correct temperature for an Aedes aegypti mosquito’s habitat will be vulnerable to dengue.

But pulling from a more complex data set that includes factors like moisture levels (the mosquitoes proliferate in wet environments) and urban populations (dengue thrives in more densely populated places), Brady and his team found a different picture.

“There’s a slight risk to Europe and the United States, not as dramatic as we would have expected,” he said. Instead, dengue will concentrate in the areas it’s already present, including Africa, Asia, and South America. Because of population growth in those places, “more of the global population will live in areas more impacted by dengue.”

Regardless, climate change and urbanization will help the spread of the disease in other ways.

Shresta explained that when a virus mutates, it learns to transmit more efficiently and survive for longer. In higher temperatures, the virus replicates – and thus mutates – more quickly. Combine that with an increasing mosquito population as temperatures warm, and there are a lot of opportunities for the virus to improve itself.

In addition, more than half of the global population lives in cities today. By 2050, an estimated 2.5 billion people will join them, according to the U.N. This trend toward urbanization is a perfect fit for dengue, which likes densely populated environments and closely shared spaces. (By contrast, according to Brady, malaria seems to prefer rural environments.)

“Unplanned urbanization happens at rates that are much higher than what authorities can cope with,” especially in poorer countries, Brady said. That leads to poorly planned housing, informal drainage, and an increase in water-holding spaces that are ideal breeding grounds for the Aedes aegypti mosquito.

In addition to distributing mosquito repellant, and using coils and sprays, the Marshall Islands’ Ministry of Health and Human Services has been working to prevent the spread of dengue by teaching people to carefully clean up their property and get rid of anything that might hold standing water.

But while he’s hopeful that the outbreak can be contained, Niedenthal has also resigned himself to a “new normal” in which contagious diseases continually make their presence known. In the low-lying Marshall Islands, “climate change does not necessarily just mean rising seas,” he said. “It’s disease.”