The city of Wuhan, China, is racing to contain the potential spread of a deadly new strain of virus that has now infected nearly 450 people. Over the weekend, the number of cases of the “2019 novel coronavirus” or “2019-nCoV” quadrupled — and on Tuesday, U.S. health officials confirmed the first case in the United States, in a man in his 30s who had recently traveled from Wuhan, China, to Seattle.

On Monday, a Chinese scientist confirmed that there can be human-to-human transmission of the respiratory illness.

Several U.S. airports have begun screening passengers from China to prevent the virus from spreading further. Here’s what you need to know:

What is a coronavirus?

Coronaviruses are a large group of viruses that can cause illnesses as minor as a cold, or as serious as Middle East Respiratory Syndrome (MERS) and Severe Acute Respiratory Syndrome (SARS), according to the World Health Organization. They often present with pneumonia-like symptoms.

The viruses are transmitted from animals to humans — the virus that causes SARS, for example, was transmitted to humans from a cat-like animal called a civet. But in some instances, as appears to be the case with this new strain of coronavirus, they can also be transmitted between humans.

The World Health Organization said there are multiple known coronaviruses circulating in animals that have not yet been transmitted to humans.

How did the new strain start?

The outbreak began in Wuhan, a city of 11 million people. Many of the patients have reportedly been linked to Hua Nan Seafood Wholesale Market, a large seafood and animal market in the city, according to CBS News’ Ramy Inocencio. But a rising number of people have apparently contracted the virus without exposure to the market, according to Chinese officials.

The market was closed on January 1, 2020 for “environmental sanitation and disinfection,” according to the World Health Organization.

How many people have died?

At least nine people have died from the illness, according to China’s National Health Commission.

The first patient, a 61-year-old man, died January 9. Two more patients died January 15 and January 18. The Commission announced the fourth patient’s death Monday, writing that an 89-year-old man died January 19 after he was admitted to the hospital with severe breathing difficulties a day earlier. The remaining deaths were announced Tuesday.

The Commission added that 169 patients are being treated in the local hospital. Thirty-five of those patients are in severe condition, and nine are in critical condition.

Where is it?

While the virus originated in China, cases have also been reported in Thailand, Taiwan, Japan, South Korea, and now the United States.

How is it transmitted?

It’s well-established that coronaviruses can spread from animals to humans, according to the World Health Organization. But on Monday, a Chinese official confirmed there have been cases in which this virus has spread from human to human.

State-run CCTM quoted Zhong Nanshan, a scientist at the China’s National Health Commission, as saying such transmission was “affirmative.” The scientist did not say how many cases were the result of human-to-human transmission — but in one case, a hospital patient is said to have infected 14 medical workers, reports Inocencio.

What’s being done to stop the spread? 

The World Health Organization announced Monday that it will convene an Emergency Committee on the virus this week in Geneva, Switzerland, to determine if the outbreak rises to the level of a public health emergency.

Meanwhile, the CDC has deployed about 100 workers total to screen passengers at the three major ports of airline entry in the U.S.: New York, Los Angeles, and San Francisco. Approximately 5,000 passengers from Wuhan are expected to pass through those airports in the coming weeks. The CDC announced Tuesday that it will also start screening at Atlanta’s international airport and O’Hare International Airport in Chicago.

The CDC said it has developed a test to diagnose the virus. Currently, that test must be administered at the CDC, but the organization is working to share the test with domestic and international partners.

In Hong Kong, which was ravaged by SARS in 2002 and 2003, hospitals upped their alert level to “serious” and implemented temperature checkpoints for inbound travelers.

In China, airline workers are running temperature checks on flights leaving Wuhan. Chinese scientists said Tuesday they had determined the DNA sequence of the new coronavirus, which could aid in the development of treatments, and potentially a vaccine.

But there could be a problem: Hundreds of millions of people are moving through China to celebrate the Chinese New Year, stoking fears that the virus could spread even faster.

Ramy Inocencio and Grace Qi contributed to this report.