New Jersey scientists are testing mosquitoes after a Morris County resident may have contracted malaria locally, which health officials said would be the first known instance in decades of someone being exposed while in the state.

About 100 cases of malaria are reported annually in New Jersey by people who have recently traveled. Officials said that the patient who was recently diagnosed with the illness had not traveled internationally, suggesting the possibility of a locally acquired infection.

The patient, who has not been identified, is recovering, health officials said.

The New Jersey Department of Health and the U.S. Centers for Disease Control and Prevention are working jointly to identify potential sources of infection.

Malaria is a mosquito-borne disease caused by a parasite and is widespread in many tropical and subtropical areas. A type of mosquito that spreads the disease, Anopheles, exists in New Jersey, but locally acquired cases are rare. One scenario in which that can happen involves someone who has been infected while traveling abroad and then returns home and is bitten by another mosquito. That insect can then acquire the parasite and spread it to other people.

According to the C.D.C., the United States eliminated malaria in the early 1950s. Each year, about 2,000 people return home from international travel with malaria infections. After 20 years without a report of a locally acquired case in the United States, nine cases were reported to the C.D.C. in Arkansas, Florida and Texas in 2023.

The New Jersey Department of Environmental Protection and a local mosquito-control program in Morris County, which is about 35 miles west of New York City, are collecting mosquitoes for testing.