State health officials say a wastewater sample collected March 20 from an Essex County water treatment plant showed traces of the measles virus. The New Jersey Department of Health said the plant serves parts of Bergen, Passaic, Hudson, Union and Essex counties. The test detected "wild-type measles virus," not the vaccine strain. A detection of wild-type measles virus in wastewater means that people who currently have or recently had measles may be present in the community, the NJDOH said in a statement. This could include people who live or work in the community, or those who traveled through the community, including through Newark Liberty International Airport. A subsequent sample taken from the same site on March 23 tested negative. The testing looks at untreated sewage entering the wastewater plants and detects viruses from infected people. The NJDOH said it can serve as an early warning system for infectious diseases but does not mean there is an active outbreak. The measles virus is spread through close contact with an infected individual and can also be spread through air but is not typically transmitted through wastewater, health officials said. No clinical cases in New Jersey residents have been identified in this region, the NJDOH said. The last confirmed case of measles in a New Jersey resident was in October 2025. Anyone who has symptoms of measles such as a high fever, cough, runny nose or a rash should call ahead to a health care provider before going to any medical office or emergency department.