It was a new virus that killed 12,469 and sickened more than 60 million Americans in one year.

The outbreak in 2009 introduced a strain of H1N1 flu that never left. Though it continues to circle the globe as a seasonal virus, the swine flu causes far less damage and anxiety than in its deadly first year.

Doctors and researchers see parallels between that pandemic and the threat of a new mysterious bug, the novel coronavirus, 2019-nCoV.

The pace of the new viral threat, first reported Dec. 31 from Wuhan, China, is breathtaking: 28,353 cases and 565 deaths as of Thursday. The outbreak, which sees more cases reported by the hour, is largely contained to mainland China; 265 cases have been reported elsewhere, including 12 in the USA and one death each in Hong Kong and the Philippines.

Government and public health officials are desperately trying to slow the spread of the respiratory virus with robust screening, quarantines and travel bans. As infectious disease doctors and scientists track the threat, they are studying past outbreaks to determine what the new coronavirus will do next.

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A best-case scenario is containing the virus and eliminating it from human circulation. That's what happened in 2003 with another deadly coronavirus, severe acute respiratory syndrome, known as SARS.

But many researchers said 2019-nCoV might be here to stay. Like swine flu, it could become a regular ingredient in a winter soup of respiratory bugs that cause common colds and other illnesses – only without the fanfare of travel restrictions and global commerce disruption.

“It’s very daunting to contain a respiratory virus, as we saw with H1N1,” said Dr. Amesh Adalja, a senior scholar at Johns Hopkins Center for Health Security. “This coronavirus might establish itself as one of our community coronaviruses that we contend with for some time during respiratory virus season.”

U.S. Centers for Disease Control and Prevention officials invoked the 2009 swine flu pandemic when discussing efforts to detect and contain the coronavirus. Agency officials would not speculate on when the outbreak might retreat.

Nancy Messonnier, director of the CDC's National Center for Immunization and Respiratory Diseases, said a difference this time is the ability to prepare.

"Rather than coming from abroad, that virus was on our doorstep when we recognized it," Messonnier said. "We are again seeing the emergence of a new virus that poses a very serious public health threat. This time, we do have time to prepare, and we are preparing as if this were the next pandemic."

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CDC officials predict there will be more cases here, but they're working to slow the spread by expanding tests, isolating infected individuals, tracking those in close contact with infected individuals and restricting travel.

"We have the opportunity to slow it down before it gets into the United States," Messonnier said. "We made an aggressive decision in front of an unprecedented threat. Action now had the biggest potential to slow this thing down."

'This is a rogue virus'

Experts suspect there are far more global cases than publicly reported. Last week, researchers from the University of Hong Kong estimated 75,815 had been infected in Wuhan as of Jan. 25 and projected the epidemic would double every 6.4 days.

"Independent self-sustaining outbreaks in major cities globally could become inevitable," the researchers wrote in a study published Jan. 31 in The Lancet.

The dire prediction included a caveat – little is known about the seasonality of the new coronavirus. If it is like the flu, which mainly circulates in the fall and winter, the Hong Kong researchers reported, "our epidemic forecast might not be reliable."

If it becomes a seasonal bug, infectious disease doctors say its spread could be limited during spring and summer months when respiratory viruses such as the flu or other lesser-known coronaviruses fade because of heat and humidity.

“Could it be that the change of seasons has an impact on the virus? Lord knows, we hope so,” said William Schaffner, a Vanderbilt University Medical Center professor of preventive medicine. “It happens with human coronaviruses, we know that. But this is a rogue virus.”

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'New virus is not SARS'

Infectious disease doctors and scientists are gathering basic evidence about 2019-nCoV. They suspect the virus jumped from an animal to a human and can spread from human to human, but unknowns remain, including how deadly it is and if it can be spread by infected people with no symptoms.

About 2% of those infected with the Wuhan coronavirus in China have died, but that fatality rate might drop as testing reveals more mild cases. Reports from Chinese government authorities suggest older individuals with underlying health conditions face greater risk. SARS, which also originated in China and spread globally into 8,098 cases and 774 deaths, had a fatality rate of 9.6%.

Though SARS was more deadly than the new coronavirus, the outbreak was far more limited. It first appeared in southern China in November 2012, but a delay in government reporting meant a global alert by the World Health Organization wasn't issued until March 2013. After spreading to two dozen countries, the outbreak was declared over four months later. No cases have emerged since 2004.

Rachel Graham, a University of North Carolina Gillings School of Public Health professor, said SARS was easier to detect because symptoms were more severe and stood out from other seasonal illnesses.

"It was easy to control, easy to eradicate from current human circulation," Graham said. "This new virus is not SARS. It's related, but it's a different virus."

Adalja of John Hopkins, who called the 2019-nCoV outbreak severe and "explosive," sees similarities between the new virus and less-heralded coronaviruses that spread more swiftly and had a lower fatality rate than SARS.

Coronaviruses 229E and OC43 have been around since the 1960s and cause common cold infections. Two others identified after SARS, NL63 and HKU1, spread during winter months, causing common respiratory illnesses such as bronchitis and pneumonia. The endemic bugs are rarely tested in patients because they are so common during the season.

Adalja said 2019-nCoV might take a similar course.

"These are other coronaviruses that were considered novel that have established themselves as part of our seasonal repertoire," he said. "This seems to be something that may follow that pattern."

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'It is ripping through the country very rapidly'

Lessons from the past aren't helping the government and industry figure out what to expect from this year's coronavirus.

Early Tuesday, some experts were hopeful China could get back to work Monday, the end of an extended holiday for workers in two dozen provinces. Such a move would signal the virus was beginning to be contained, but by the end of the day, that hope had faded.

"Most pandemic experts believe it's no longer in the cards," said Stephen Morrison, senior vice president of global health policy for the Center for Strategic and International Studies. "It is ripping through the country very rapidly."

Morrison said the lack of certainty has made it difficult for airlines and other businesses in China to plan.

"They are all, to some degree, trying to read the scenarios and the timelines," he said.

The U.S. Chamber of Commerce said the most important thing is to contain the virus.

“U.S. companies are taking responsible steps to address the threat and doing everything in their power to support relief efforts in China," the chamber said in a statement to USA TODAY. "The U.S. government is also meeting around the clock, and we trust that they are taking the appropriate steps to prioritize the safety and security of all Americans.”

Schaffner of Vanderbilt said much relies on China's effort to contain global spread. Its quarantine of Wuhan, a city of 11 million, and travel restrictions amount to "the largest public health experiment in the history of public health."

"As draconian as it was, it may have some effect," Schaffner said. "A lot depends also on identifying people and getting them into medical care. Will the public health responses be vigorous in those locations, or are we going to get subsequent outbreaks ... in other parts of China? Those unknowns are huge at the moment."