The CDC confirmed two new cases in Southern California and one in Maricopa, Arizona. All five patients in the U.S. had traveled to Wuhan, China, officials say.
A person with the Wuhan coronavirus could be infectious to others before they know they have the virus, according to China's National Health Commission Minister Ma Xiaowei.
At a Sunday press conference, Ma said someone with the virus could show no symptoms they have the coronavirus anytime between a day and 14 days, CBS News previously reported. During that time, an asymptomatic person is also infectious and could spread the coronavirus to others, Ma said, which makes it difficult to stop its spread.
Coronavirus symptoms include nasal congestion, headache, cough, sore throat, and a fever, according to infectious disease expert and University of California San Francisco professor Charles Chiu.
"At present, the rate of development of the epidemic is accelerating," Ma said at the press conference. "I am afraid that it will continue for some time, and the number of cases may increase."
The virus, which originated in Wuhan, China, has killed 56 people and infected more than 2,000, Reuters reported on Sunday.
The toll so far As of Tuesday, more than 4,500 cases have been confirmed in China, its country of origin, of which 106 have been fatal. Several cases have been detected in Asian countries, as well as a few in Australia, France and the United States.
So far, no-one outside China has died, but both Germany and Japan on Tuesday confirmed cases of human-to-human transmission of the disease.
2019-nCoV, as it has been named, is part of the coronavirus virus family, the source of two previous fatal epidemics.
The 2002/03 SARS outbreak (Severe Acute Respiratory Syndrome) started in Beijing and killed 774 people out of a total 8,096 infected.
The 2012 MERS outbreak (Middle East respiratory syndrome) killed 858 people out of the 2,494 infected.
So those outbreaks had significantly different mortality rates of 9.5 percent and 34.5 percent respectively.
In general however, those hit by the new virus "are in a less serious condition than with SARS", said Yazdan Yazdanpanah, head of infectious diseases at Paris Diderot University.
"For the moment, the mortality rate is less than five percent", he added.
Symptoms Chinese scientists reported in The Lancet Friday that, based on a study of 41 early-detected cases, some of the new virus's symptoms resemble those of SARS.
All patients had pneumonia, most had a fever, three-quarters of them were coughing and more than half had trouble breathing.
Despite this, lead author Bin Lao added: "there are some important differences", such as a lack of runny noses, sneezing or sore throats.
The average age of the 41 patients studied was 49, most of them having visited Wuhan market, identified as the source of the outbreak. Nearly a third of them had serious breathing difficulties and six of them died.
All this gives a preliminary sketch of the new virus, even if one has to be cautious about drawing conclusions based on such a small sample.
The study is all the more important because a current epidemic of flu, which has similar symptoms, has made isolating patients of the new virus difficult.
Contagion Scientists at Britain's Imperial College estimate that each coronavirus patient infects on average 2.6 others - making it roughly as infectious as annual influenza outbreaks.
Ma Xiaowei, head of China's National Health Commission, said that transmission of coronavirus was possible during the disease's incubation period.
That means that someone who is sick with coronavirus would be able to pass it on to someone else even if they aren't yet displaying any symptoms.
This working hypothesis is yet to be fully confirmed, however.
"In my view it is premature to conclude, on the basis of the evidence currently available, that the new virus can be transmitted before symptoms appear," said Mark Woolhouse, professor of Infectious Disease Epidemiology, University of Edinburgh.
Origins Researchers think the new virus probably came from bats, as the SARS virus did, with which it shares 80 percent of its genetic makeup.
But we still don't know which animal passed it on to humans. Last week a Chinese team suggested it could be a snake, but that was immediately challenged by other experts, who think a mammal is the more likely culprit.
Identifying which animal it was could help fight the outbreak.
With SARS, it turned out to be a civet, whose meat is very popular in China.
Banning the consumption of civet and closing their breeding farms helped stop SARS from making a comeback, says Arnaud Fontanet of the Pasteur Institute in Paris.
On the other hand, one reason it was harder to stem the MERS outbreak is because it came from dromedary camels, a widely used working animal.
The mysterious source of the novel coronavirus outbreak, which originated in Wuhan, central China and since spread globally, has led to a significant divide among scientists worldwide.
While a team of scientists has raised doubts that the virus originated from Wuhan's Huanan Seafood Market, another group of researchers claims snakes sold in the market led to the outbreak. These conflicting findings has led to heated debate among researchers.
Finding the source of the outbreak is crucial to preventing and controlling future outbreaks of the virus. The origin of the novel coronavirus is still unknown, but it's most likely an animal reservoir, said the World Health Organization (WHO).
Theory 1: The seafood market may not be the source
A comprehensive study led by a team of more than 20 Chinese researchers analyzed data from 41 patients diagnosed with the new virus, known by its technical name 2019-nCoV, at a hospital in Wuhan.
They found only 27 out of 41, or 66 percent of patients, had direct exposure to Huanan Seafood Market. The rest of the admitted patients had no exposure to the market.
Since the first case of the virus was identified on December 1 and admitted to the hospital, "none of his family members developed fever or any respiratory symptoms," said researchers.
"No epidemiological link was found between the first patient and later cases," researchers wrote in the study published in the Lancet journal.
The Wuhan government suspects the source of the outbreak was the market and closed it down on January 1. A range of animals and pre-packaged meat on sale were removed in order to control the virus from spreading.
Theory 2: Snakes as the intermediary host
But another research team led by researchers from Peking University claims snakes – Chinese krait and the Chinese cobra – sold along with other wild animals at the Huanan Seafood Market, were a possible host of the new coronavirus.
They analyzed the protein codes favored by the new coronavirus and compared it to the protein codes from coronaviruses found in different animal hosts, like birds, snakes, marmots, hedgehogs, bats and humans.
The coronavirus family also causes severe acute respiratory syndrome (SARS) and Middle East respiratory syndrome (MERS), usually found in bats, requires an intermediary host – palm civets in the case of SARS and camels for MERS – to infect humans.
"Snakes were also sold at the Huanan Seafood Wholesale Market where many patients worked or had a history of exposure to wildlife or farm animals," said the study accepted for publication in the Journal of Medical Virology.
"Taken together, [bats] and snakes could be the most probable wildlife animal reservoir for the 2019-nCoV."
The study concluded that snakes were an intermediary host of the virus jumping on to humans, which has killed 107 and infected more than 4,400 people as of January 28.