NEW YORK (AP) – The United States has seen five cases of malaria spread by mosquitos in the last two months—the first time there has been local spread in 20 years.

There were four cases detected in Florida and one in Texas, according to a health alert issued Monday by the Centers for Disease Control and Prevention.

Malaria is caused by a parasite that spreads through mosquito bites. Infected people can suffer fever, chills and flu-like illness. If it goes untreated, infected people can develop severe complications and die. The largest death toll in recent years has been seen in children in sub-Saharan Africa.

Health officials are warning doctors, especially those in southern states where the weather is more friendly to the tropical mosquito that spreads malaria, to be aware of the possibility of infection. They also should think about how to access the IV drug that is the first-line treatment for severe malaria in the United States, the CDC said.

The agency said that the people who were diagnosed received treatment and “are improving.”

About 2,000 U.S. cases of malaria are diagnosed each year—the vast majority in travelers coming from countries where malaria commonly spreads.

Since 1992, there have been 11 outbreaks involving malaria from mosquitoes in the U.S. The last one occurred in 2003 in Palm Beach County, Florida, where eight cases were reported.