The United States has issued a nationwide health alert for the first time in 20 years over the spread of malaria caused by mosquitoes, as five cases were seen in two states, Texas and Florida, in the last two months.

Four people in Florida and one person in Texas contracted the disease.

The Centre for Disease Control and Prevention issued a health alert on malaria spread on Monday.

The agency assured citizens that those diagnosed with malaria are receiving treatment and “are improving,” while health officials warned citizens to take precautions and spray insecticide within their environment.

“Avoid areas with high mosquito populations, and wear long pants and shirts when possible, especially during sunrise and sunset when mosquitoes are most active,” it stated.

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.

The U.S. records 2,000 malaria cases yearly, but those diagnosed are usually travellers from countries where malaria is endemic.