Six Americans — in California (2), Arizona, Washington state, and Chicago (2) — have the novel coronavirus that's been spreading from China, as of Thursday according to the Centers from Disease Control and Prevention.

What they're saying: "We need to prepare as if this is a pandemic, but we hope it is not," Nancy Messonnier, director of CDC's National Center for Immunization and Respiratory Disease told a press conference on Sunday.

"The risk to Americans is low at this time but the threat is serious and the public health response is aggressive."

The latest:

The CDC confirmed Thursday the first person-to-person transmission of the coronavirus in the U.S. Officials said a Chicago woman who traveled to Wuhan, China, the center of the outbreak, spread it to her husband.

While U.S. public health officials have seen no evidence that the virus is infectious before a person shows symptoms — something that would likely call for stronger screening programs — they are investigating reports from two recent studies that point to that possibility.

By the numbers: There are 92 people under investigation in 36 states, Messonnier says.

"We expect there to be additional cases of coronavirus in the United States," she says.

What's next: The CDC plans to update its numbers on Friday.

Public health officials are working to develop specific diagnostics, therapies and vaccines for this particular virus. They've isolated the genetic sequence from the first patient in the U.S. and are comparing this to the first patient found in China to see how it's evolving.

Background: Coronaviruses are zoonotic, which means they can transmit from animal to human and then have the ability to evolve so they can transmit between humans. They range in severity from causing the common cold to deadly diseases like SARS and MERS.

SARS, or Severe Acute Respiratory Syndrome, was believed to have originally transmitted from civet cats to humans, although recent genetic testing shows bats are the natural hosts. It was first reported in China in 2002 and recognized as a global threat in 2003, resulting in 774 deaths and serious economic impact. It mysteriously disappeared, for now.

MERS, or the Middle East Respiratory Syndrome, is believed to have originally transmitted from dromedary camels to humans. It was first reported in Jordan and Saudi Arabia in 2012, had two U.S. cases in 2014, and it continues to circulate mostly in the Middle East where the CDC is monitoring it.

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