West Nile virus, a mosquito-borne sickness that is transported by infected birds, reemerged this week for the third year since it was first reported in the United States in New York City in summer 1999. The virus, which had not been seen since last fall, was found in two dead crows in New Jersey, officials announced Monday.

Scientists still aren't sure how it got here in 1999. That year, it killed seven people and made 62 seriously ill, mostly in the New York City area. The virus, which was first seen in Uganda in 1937, does not sicken most of the people it infects. But in those who are old and infirm, or have weakened immune systems, the virus can cause encephalitis, a potentially deadly swelling of the brain.

Meanwhile, the state has activated its West Nile virus hot line, 866-866-CROW, to help inform people about the virus and to provide instructions about handling possibly infected birds.