The New York and San Francisco airports each receive three direct flights from Wuhan each week, said Dr. Martin Cetron. Los Angeles International gets significant numbers of passengers who start their journeys in Wuhan but change planes in Beijing.

Some, but not all, of the cases appear to be linked to a large seafood market in the city, which also sells live animals for meat. Experts think the infection probably came from people touching or eating animals that carry the virus. These individuals then developed viral symptoms including fever, breathing issues and lesions on their lungs.

The coronavirus family includes six other strains known to infect humans. All share a signature look under a strong microscope: a circle with spikes coming off the surface, ending with small blobs — hence the "corona."

"Kind of looks like the peaks of a crown," says Carolyn Machamer, a virologist and cell biologist at the Johns Hopkins University School of Medicine.

Four of those strains cause common colds, and two — SARS and MERS — have caused major pandemics. So how concerned should you be about this new disease?

Virologists watching the outbreak are asking three main questions to assess the severity of this coronavirus, on the scale of sniffles to severe threat:

1. Is it Contagious?

Chinese authorities say the virus does not transmit easily between people.

Since health workers treating patients don't seem to have fallen sick, it's reasonable to believe that the virus doesn't spread efficiently, says Vineet Menachery, a virologist at the University of Texas Medical Branch at Galveston.

The ease of person-to-person spread rests on several factors, including how well the virus replicates in a person and where in the body the infection is located.

"If it's in the nose or the upper airways, it potentially could be breathed or coughed out easier" compared with a deeper lung infection, says Matthew Frieman, a virologist at the University of Maryland School of Medicine.

The SARS virus spread readily through close person-to-person contact. The SARS pandemic originated in southern China in 2002, infected more than 8,000 people globally and caused some 800 deaths before the outbreak ended the following year.

MERS, which emerged in 2012 in Saudi Arabia, appears to spread less easily between people but to be more lethal. To date, it has infected around 2,500 people and caused around 850 deaths.

2. Is it Lethal?

Chinese health authorities have reported two deaths from the virus in Wuhan: a 61-year-old man, described as a frequent visitor to the market, died last week, and a 69-year-old man, who became ill on New Year's Eve, died Wednesday.

About half-a-dozen patients were in critical condition. Other patients have been treated and discharged.

The fact that people seem able to fight off the infection and recover is a good sign, says Machamer of Johns Hopkins.

"It's a serious-enough infection, but not like SARS or MERS," she says.

Health authorities are monitoring the patients in Wuhan and their close contacts to help manage the outbreak.

3. Is it Contained?

So far, three cases of coronavirus have been confirmed outside of China, in travelers from Wuhan. Two Chinese tourists in Thailand and a Japanese man who returned from a trip to Wuhan were all confirmed to have the coronavirus. The World Health Organization says it's "not unexpected" for cases to be found elsewhere and called on other countries to monitor for signs.

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