The Centers for Disease Control and Prevention has issued a health alert after five cases of malaria were detected in two states, the first locally transmitted cases in the U.S. in 20 years.

Four cases have been reported in Florida and one in Texas in the last two months and concern over a potential rise due to imported cases is increasing with summer travel, the CDC said. The Texas and Florida cases do not appear to be related to each other but the four Florida cases are all in geographic proximity to each other, prompting additional surveillance in the area.

All of the patients have been treated and improving, according to the CDC.

The CDC advisory urges physicians to consider a diagnoses of malaria in “any person with a fever of unknown origin,” especially if they live in an area with locally acquired malaria.

Locally acquired mosquito-borne malaria has not occurred in the U.S. since 2003 when 8 cases were identified in Palm Beach County, Florida.

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Malaria is a disease spread when the female anopheline mosquito feeds on a person with malaria and then feeds on another.

Malaria is a serious and potentially fatal disease that’s transmitted when a female anopheline mosquito feeds on a person with malaria and then bites an uninfected person. Worldwide, more than 240 million cases of malaria occur each year, almost all in Africa. U.S. cases have traditionally been imported via international travel.

About 2,000 of travel-related malaria are reported in the U.S. each year, according to the CDC.

Symptoms include fever, chills, headache and fatigue, as well as nausea, vomiting and diarrhea. Symptoms typically being 10 days to 4 weeks after infection and, if not treated, can lead to severe disease that includes mental changes, seizures, renal failure, acute respiratory distress and coma.

You can go here to see CDC recommendations for protecting yourself against malaria.