The Health Department confirmed Thursday that West Nile virus has returned to New York City during the closing days of summer, with four people testing positive for the mosquito-borne illness.

“The findings from our mosquito and human surveillance serve as a reminder for all New Yorkers that they should take simple precautions to protect themselves and their families from mosquito bites,” Health Commissioner Dr. Oxiris Barbot said in a statement.

Two of the cases are in Queens, while Brooklyn and Staten Island have each seen one case.

In people over 50 or with a weakened immune system, West Nile virus can cause severe illness, including meningitis and encephalitis, sometimes resulting in permanent or long-term complications such as muscle weakness, fatigue, confusion and depression. Others may experience milder symptoms, which include headache, fever, fatigue and rash, the Health Department said in a release.

The virus was first detected in New York City in 1999 and has returned every summer since. Last year, 36 New Yorkers contracted the disease. Since 1999, the number of human cases has ranged from three to 47 annually. Thirty-six New Yorkers were diagnosed with West Nile virus during the 2018 season.

Officials say New Yorkers can cut down their risk for infection by wearing pants and shirts with sleeves, using mosquito repellent when they go outside and making sure their windows have screens.