Officials said if the public and health-care workers are aware of the symptoms of serious illness, they can be ready to treat those at risk of the deadly encephalitis and meningitis that the virus can cause.
Last year was the first time the West Nile virus was ever detected in North America, reports CBS News Correspondent Jim Axelrod.
Steady testing over the winter months has shown virus-carrying mosquitoes have survived the cold season. The worry now is that the virus will spread.
Dr. Stephen Ostroff, West Nile virus coordinator for the U.S. Health and Human Services Department (HHS), said, "Eliminating it from the New York metropolitan area, or the Western Hemisphere, is probably unrealistic."
That's because mosquitoes bite birds too, which can then transmit the virus to other mosquitoes as the birds migrate. The CDC has started watching 17 states from New England to Texas for signs of West Nile.
The virus was first discovered late last summer - too late for anything but an all out aerial spray assault using helicopters.
This year it's a preemptive ground strike, hoping to kill young mosquitoes this spring before they mature into flying killers later this summer.
New York launched an aggressive campaign against the mosquitoes earlier this month, spraying insecticides, stocking bodies of water with fish that eat mosquito larva, keeping an eye on birds that might become infected and educating people about getting rid of stagnant water, using bug repellent and keeping screens up.
The virus has been familiar for decades in Africa and Asia, but had never been seen in the Americas before last year.
Ostroff said it took U.S. health officials by surprise.
"We know that when this problem came out this fall that many partners at the state and local level were unprepared," Ostroff said. He said programs aimed at controlling and responding to such outbreaks had deteriorated, and said coordination between federal and local officials was vital.
He said HHS, the Centers for Disease Control and Prevention (CDC), Environmental Protection Agency (EPA) and other agencies would work together this year to try to control the virus as much as possible.
He said West Nile was yet another example of newly emerging infectious diseases that still threaten Americans in the age of modern medicine. AIDS is one of the biggest examples but hantavirus, a deadly virus carried by rodents, is another.
"It highlights our vulnerability to emerging infectious diseases, when we live in an era of global travel, global change and environmental change," he said.
Scientific study of the virus shows it is most closely related to a strain found in Israel, Ostroff said.
"The issue of how it may have moved from Israel to the New York metropolitan area is still open to a great deal of debate," he said.
He said no one might ever know whether it was carried by a bird, by a mosquito that somehow got aboard an aircraft or ship, or whether an infected person brought it.
Most people infected with West Nile virus get a mild, flu-like illness that has no severe symptoms. Only a few develop the brain inflammation that can kill.
Ostroff said HHS helped the New York City Department of Health survey the north Queens area hardest hit by the virus. Anonymous blood samples given by residents suggested about 2.5 percent had been infected - most of them without symptoms.
He said he believed fewer people elsewhere in the city would have been infected.
The best way to control the virus is to prevent its spread by controlling mosquitoes, Ostroff said. "There definitely is a role for the public to help us in our control action to help us eliminate potential breeding sites around the home in particular," he said.