Cases of the virus have now been reported in 12 states -- the majority of them in California and Washington state, where six people have died.
About a quarter of the current cases were likely transmitted through US communities, officials have said, meaning they were not travel-related.
"My concern is as the next week or two or three go by, we're going to see a lot more community-related cases," Dr. Anthony Fauci, the director of the National Institute of Allergy and Infectious Diseases, said in a news conference Monday. "That's of great concern."The number of cases has been climbing as new guidelines from health officials gave more labs the go-ahead to conduct tests. Faulty kits from the Centers for Disease Control and Prevention initially led to delayed and inconclusive results.
Over the weekend, the US Food and Drug Administration further expanded who could test for the virus by allowing additional labs to develop their own tests for the virus. The move, FDA Commissioner Dr. Stephen Hahn said, would enable academic centers and private companies to develop and use tests.
He said up to a million tests will likely be conducted by the end of this week.