The discovery of the spread of the virus by researchers, based on a genetic analysis of virus samples, has sobering implications amid heightening anxiety about the probable spread of the disease. The researchers conducted genetic sequencing of two virus samples. One is from a patient who travelled from China to Snohomish County, north of Seattle, in Washington state in mid-January and was the first person diagnosed with the disease in the United States. The other came from a recently diagnosed patient in the same county, a high school student with no travel-related or other known exposure to the coronavirus. The two samples look almost identical genetically, said Trevor Bedford, a computational biologist at Fred Hutchinson Cancer Research Centre in Seattle who announced the results of the research on Twitter late on Saturday night.

Loading Replay Replay video Play video Play video "This strongly suggests that there has been cryptic transmission in Washington State for the past 6 weeks," Bedford wrote. "I believe we're facing an already substantial outbreak in Washington State that was not detected until now due to narrow case definition requiring direct travel to China." Officials in Seattle and King County on Sunday announced that two more people have tested positive for the coronavirus, which causes the disease named covid-19. Both patients are men in their 60s and are in a critical condition.

Health officials in Washington state and across the nation said they expect numbers will continue to rise in the wake of the decision by the US Centres for Disease Control and Prevention last week to widen testing guidelines. Over the weekend, new cases were reported in Americans who had recently travelled to South Korea and Italy, including one person in Rhode Island, the state's first case.

The health department in Santa Clara County, California, in the heart of Silicon Valley, announced three new coronavirus cases on Sunday evening, bringing to seven the total number of cases there. The announcement gave few details about the cases. Robin Addison, a nurse at Providence Regional Medical Centre, in Everett, Washington, the state where coronavirus cases have emerged. Credit:AP California and Oregon, like Washington, have reported coronavirus infections in people who did not travel to regions hit hard by the outbreak or have contact with people known to be infected. The United States has dozens of other confirmed infections, the majority of them people who were among the passengers on the cruise ship Diamond Princess. The international picture has continued to worsen, with spikes in cases in South Korea, Iran and Italy in recent days. Worldwide, almost 90,000 people have been infected with the coronavirus, and 3000 have died. The research reported by Bedford is preliminary, and further analysis could alter the conclusion.