The U.S. will begin screening Friday night for the novel coronavirus, originally found in Wuhan, China, for flights arriving directly or indirectly from there to three American airports — San Francisco, New York and Los Angeles, public health officials announced.Why it matters: Researchers still don't know the source of the "2019-nCoV" virus or how it's transmitted, but coronavirus' ability to evolve means the outbreak could quickly turn from "worrisome to extremely worrisome," and "proactive measures" should be taken, Nancy Messonnier told a press conference.Background: The virus was originally reported Dec. 30 by Chinese health officials in Wuhan — a large city with more than 11 million people.There are more than 40 known cases of the virus, which has killed two people and spread to at least two more countries, with two cases in Thailand and one in Japan from people who had been in Wuhan.Chinese health authorities fully sequenced the virus and the data is now in the NIH's database GenBank and in the Global Initiative on Sharing All Influenza Data (GISAID) portal.The CDC and other global health organizations have urged health officials in Wuhan to begin exit screening at their airports to look for possibly infected people before they board an airline. Just before the press conference ended, Martin Cetron, director of the CDC's division of global migration and quarantine, said new reports indicate this may have already started.The latest: Starting Friday night, the CDC and the Department of Homeland Security’s Customs and Border Protection (CBP) will implement enhanced health screenings to detect ill travelers en route to the U.S. on direct or connecting flights from Wuhan to SFO, JFK, and LAX airports.Passengers will receive a questionnaire asking about symptoms and will have their temperatures taken. If there are concerns, the passenger and family members will be taken to a separate facility for further testing and care.The CDC has testing available based on the virus' genome, but should "imminently" have new diagnostics available, Messonnier, director of the National Center for Immunization and Respiratory Diseases, adds.What we don't know: Researchers are trying to determine where the virus originated, if it can be transmitted from person-to-person, and how long the incubation period may be.Per the CDC, most Wuhan patients are linked to a large seafood and animal market, "suggesting animal-to-person spread." But, others did not have this exposure, which suggests "some limited person-to-person spread may be occurring."Coronaviruses are particularly tricky because they are found in animals and humans, and the cases where animal-to-human infections lead to human-to-human infections can be severe, as seen in SARS and MERS coronavirus infections.The bottom line: "It's highly plausible there will be a case in the U.S. and that's why we are moving forward to new screening," Messonnier says.Go deeper: China virus kills one and spreads to Thailand, but U.S. threat remains minimalChina hunts cause of mysterious pneumonia outbreak in Wuhan