For the first time in two decades, malaria infections have been confirmed in people who did not travel outside the United States, leading federal health authorities to warn about the potential for transmission of the mosquito-born disease within the nation’s borders.

Four people in Sarasota County, Fla., and one in Cameron County, Tex., were confirmed as having been infected between late May and late June through local transmission. All have gotten treatment and are recovering as health officials watch for additional cases, the Centers for Disease Control and Prevention said.

Although the potentially fatal disease was once endemic, it was declared eliminated in the United States in 1951. About 2,000 people are diagnosed with malaria in the nation each year, but those cases have involved trips abroad. For a handful who came down with the disease in recent months, that was not the case.

The risk of getting malaria in the United States “remains extremely low,” the CDC said. Still, experts said Americans should be aware of the possibility and take steps to prevent mosquito bites.

“It’s not panic time,” said Brian Grimberg, an associate professor of pathology and international health at Case Western Reserve University. “I think the message is to be aware. I mean, Americans never think about malaria unless they travel abroad.”

Malaria is a serious disease with symptoms including fevers, headaches, chills and flu-like illness. Around the globe, more than 240 million infections happen each year – 95% of them in African countries.

In the United States, where malaria was once a major public health threat, the CDC was created to combat the spread of the disease. Through the use of the insecticide DDT, draining of swamps and other measures, Grimberg said, those efforts were largely successful and malaria retreated as a concern.

The last confirmed instance of local transmission happened in 2003, when eight people became infected in Palm Beach County, Fla., the CDC said.

In the five cases discovered between May and June, the species of malaria parasite has been identified as P. vivax, which is less likely to cause severe disease. But failing to treat it can cause relapsing episodes.

Malaria is treated using medication that is generally widely available in the United States. Health officials advise that people who suspect they have it get evaluated, diagnosed and treated soon after symptoms arise.

“If appropriately treated, you can recover quickly,” Grimberg said.

To lower the risk of getting malaria and other mosquito-borne diseases, the CDC is urging the public to apply insect repellent, use screens on windows and doors, and drain items that hold water – such as tires, bird baths, buckets and trash containers – at least once a month.

Overseas travelers should pack bug spray, stay in places with air conditioning or window and door screens, or sleep under a mosquito net.

The agency is also recommending that hospitals have malaria tests available and stock up on treatments, and that public health officials have a plan for rapid identification, prevention and control.