Opinion | I’m a Doctor. Dengue Fever Took Even Me by Surprise on Vacation.

Two days later, the test was positive.

Despite my training in medicine, I was blindsided. Dengue, a mosquito-borne illness, is surging through Latin America and the Caribbean, including in Puerto Rico, where a public health emergency was declared last week. This year is likely to be the worst on record, in part because of El Niño-driven temperature spikes and extreme weather linked to climate change. As temperatures rise and precipitation patterns grow more erratic, the problem will get only worse.

But neither the traveling public nor our frontline health workers are prepared. Without urgent reforms to how we educate travelers, doctors, nurses and others — as well as reforms to public health surveillance and early warning systems — we will be doomed to miss textbook cases like mine. That means those infected with dengue will miss out on timely treatment, possibly even spreading the virus to areas where it was never found before.

The dengue virus, which is primarily transmitted by the Aedes aegypti mosquito, infects up to 400 million people every year in nearly every region of the world, but it is most prevalent in Latin America, South and Southeast Asia and East Africa. Most cases are asymptomatic or, like mine, are considered mild, although the aptly nicknamed breakbone fever often doesn’t feel that way. Some 5 percent of cases progress to a severe, life-threatening disease including hemorrhagic fever.

One malicious feature of dengue is that when someone is infected a second time with a different type of the virus, the risk of severe illness is higher. A vaccine exists, but the Centers for Disease Control and Prevention recommends it only for children ages 9 to 16 who had dengue before and live in places where the virus is common. That’s because, paradoxically, if you’ve never had dengue, the vaccine puts you at greater risk of severe illness your first time.

Dengue outbreaks, which, in the Americas, tend to occur every three to five years, now appear to be expanding their geographic reach as temperatures climb. The Aedes aegypti mosquito has typically had difficulty surviving and reproducing during the winter in temperate climates. But in parts of Brazil, which is experiencing a dengue emergency, the thermometer no longer dips as low in the winter as it once did, allowing the bugs to reproduce year-round. Overall, Latin America and the Caribbean have had three times the number of cases this year as reported for the same period in 2023, which was a record year. Higher temperatures are also helping the virus develop faster inside the mosquito, leading to a higher viral load and a higher probability of transmission. And mosquitoes are benefiting from standing water from rains and floods that are growing more extreme in a warming world.