The Zika virus should not cause Americans to be "unduly alarmed just yet," the co-director of the Georgetown Center for Infectious Disease told CNBC on Friday.
"[There's] no local transition on our shores just yet," Paul Roepe told CNBC's "Squawk Box" a day after the World Health Organization said the mosquito-transmitted disease, linked to severe birth defects in thousands of babies in Brazil, was "spreading explosively."
The WHO, promising quick action after last year's criticism about reacting too slowly to West Africa's Ebola outbreak, said Zika could infect as many as 4 million people in the Americas.
"The highest level of virus in your system only lasts for a couple of days and then dissipates. So the viral load goes way, way down. Probably most of these infections self-resolve ... in about a week of so," said Roepe, a biochemistry professor at Georgetown University.
There's no vaccine or treatment for the virus, which is like dengue and causes mild fever, rash and red eyes. An estimated 80 percent of people infected show no symptoms.
"The only way you can get Zika is by the bite of an infected mosquito. And that mosquito has to pick it up from an infected person," Roepe said. "It's spreading quickly in Central and South America because there are lot of people that are currently infected."
He said another reason those regions have been so hard hit is that Zika is more commonly passed along among the kind of mosquitoes there, a species not as widespread in the United States.
But he warned that a "larger concern" could develop in the U.S. if transmission became prevalent among a related type of mosquitoes that is routinely found in the Southeast and even as north as New York during warmer months.
U.S. health officials said Thursday while they have not yet seen spread of the disease in the 50 states, the number of U.S. travelers infected over the last year in the Caribbean or Latin America has climbed to 31.
The Zika virus was discovered in Africa in 1947. But until last year, when it was found in Brazil, it had never been a threat in the Western Hemisphere.
WHO officials are set to convene a special meeting next week to decide whether they should declare an international health emergency.