In the 1950s, the U.S. declared that it had wiped out malaria. As NPR reported a few years ago, this public health milestone was achieved "through the use of insecticides, drainage ditches and the incredible power of window screens."

This week, headlines trumpeted the "return" of malaria: four cases in Florida and one in Texas.

So what's going on – and how worried should Americans be?

A bit of history is in order before those questions can be answered.

For years, malaria was a serious health issue in the U.S. – in fact the Centers for Disease Control and Prevention was established in 1946 to quash the disease. There were 15,000 cases in 1947. But by 1950 the number was only 2,000, and then came the triumphant announcement of 1951.

But malaria didn't completely disappear from the United States. Each year, there are about 2,000 cases diagnosed here. Symptoms can include high fever, body aches, diarrhea and vomiting, and the U.S. does see a few annual deaths from the disease.

Epidemiologists believe that the patients were likely travelers or immigrants who contracted the disease outside the U.S. rather than from a bite by a mosquito within U.S. borders.

There was a blip in 2003 – eight cases identified as locally transmitted in Palm Beach, Florida.

Home-grown cases of malaria

And now there are the Florida and Texas cases. In an email to NPR, the CDC stated that "according to the investigations conducted by the health departments in FL and TX, all five cases are not linked to international travel."

The CDC adds that the patients had contracted P. vivax within the last two months – a strain of malaria that typically produces milder symptoms or can even be asymptomatic but still can prove fatal, especially in those who are pregnant and in children.

"It's not benign. People can feel completely wiped out, laid up and out of commission for weeks," says Dr. Amy Vittor, an infectious diseases physician at the University of Florida in Gainesville, who has researched malaria in the Peruvian Amazon.

Any malaria case should be considered a medical emergency, notes Dr. Monica Parise, director of the Division of Parasitic Diseases and Malaria in CDC's Center for Global Health. The agency recommends that people diagnosed with malaria be hospitalized, at least initially, to make sure the infection is "going in the right direction."

"It starts out mild, but it can progress to very severe disease and death if it's not promptly treated," Parise says.

The disease claims an estimated 600,000 lives a year around the world.

The location of these cases is not surprising. Florida's temperate climate has always been hospitable to mosquitoes, including the species that carry malaria. "A lot of Florida is suitable for malaria transmission for much of the year," says Sadie Ryan, a professor of Medical Geography at the University of Florida and Director of the Florida Climate Institute.

Texas is also no stranger to mosquito-borne illnesses and saw a Zika outbreak in its southern region in 2016. Texas "is certainly within suitable temperature thresholds for transmission" of malaria, says Ryan.

So how did mosquitoes in the U.S. come to carry the malaria parasite?

One possible explanation has to do with the nature of the P. vivax parasite. Because the malaria it spreads is typically mild or asymptomatic, there might be Floridians and Texans who were infected while traveling and "never knew they had malaria," says Ryan. "It's possible that mosquitoes picked it up" from these oblivious travelers and then did what mosquitoes do – bit people and spread the parasite to others in the U.S.

The pandemic might have played a role in detection of these cases, suggests Matt Hitchings, a professor at the University of Florida's Center for Statistics and Quantitative Infectious Diseases. Some symptoms of malaria are the same as COVID-19 symptoms: high fever and body aches. With more people seeking medical attention for flu-like symptoms, along with the general awareness of mosquito-borne illnesses in Florida, these malaria patients could have come forward.

But without more details, experts say it's impossible to offer definitive explanations. Rhoel Dinglasan, director of the CDC's Southeastern Regional Center of Excellence in Vector Borne Diseases at the University of Florida, says that to know where the parasite came from, scientists would need to sequence the DNA of the parasite from these specific cases. From there, "we could look at the relatedness of this sequence" to other parasites from around the world, says Dinglasan.

How does climate change affect the spread of malaria?

Then there's the climate change question – were warming temperatures or increased rainfall (which creates standing water in which mosquitoes breed) part of the story?

Of course, malaria-carrying mosquitoes have been active in the U.S. before.

But there is concern that hotter temperatures around the globe could lead to wider spread of malaria. Mosquitoes breed best in a warm climate – and are knocked out by cold, which also keeps the malaria parasite from reaching full maturity in the mosquito.

So longer and warmer summers could make certain parts of the U.S. more hospitable to malaria-infected mosquitoes, says Dinglasan.

Still, scientists say it's too soon to tell if these specific cases in the U.S. are related to climate change.

The same caution applies to malaria cases around the world.

According to the World Health Organization, malaria cases globally rose slightly from 245 million cases in 2020 to 247 million cases in 2021. According to Pedro Alonso, former director of the Global Malaria Programme at the World Health Organization in Geneva, the rise in cases could be due to a combination of reasons, including difficulty getting bed nets to affected places during the pandemic and global complacency.

The World Health Organization believes that we could soon see 250,000 additional deaths per year from malaria, malnutrition and heat stress due to climate change. Martin Edlund, the CEO of Malaria No More, notes that climate disasters and extreme weather events could also disrupt routine health care, especially in poorer countries, making it harder to treat diseases like malaria. "We should really double down on ending malaria globally," says Edlund. "If we can take one of the biggest climate-sensitive disease risks off the table," then we could save lives globally and "protect us here at home."

So while scientists do further investigation of the U.S. situation – and look for more cases in Florida and Texas – public health experts offer advice that's not surprising but could be increasingly critical. The CDC's Rhoel Dinglasan suggests wearing insect repellent in the early evenings and nights, when mosquitoes are more likely to bite, and reporting standing water to local mosquito control agencies.

There are similar calls for better surveillance in areas of the world where malaria might previously not have been seen. As conditions become more hospitable for mosquitoes that carry malaria, "we should be concerned," says Ryan. "We should be paying attention."

NPR health reporter Pien Huang contributed to this story.