A virus – previously unknown to science – is causing severe lung disease in China and has also been detected in other countries.
More than 100 people are known to have died from the virus, which appeared in the Chinese city of Wuhan in December.
There are already more than 4,500 confirmed cases, and experts expect the number will keep rising.
A new virus arriving on the scene, leaving patients with pneumonia, is always a worry and health officials around the world are on high alert.
Can this outbreak be contained or is this something far more dangerous?
What is this virus?
Officials in China have confirmed the cases are caused by a coronavirus.
These are a broad family of viruses, but only six (the new one would make it seven) are known to infect people.
Severe acute respiratory syndrome (Sars), which is caused by a coronavirus, killed 774 of the 8,098 people infected in an outbreak that started in China in 2002.
“There is a strong memory of Sars, that’s where a lot of fear comes from, but we’re a lot more prepared to deal with those types of diseases,” says Dr Josie Golding, from the Wellcome Trust.
How severe are the symptoms?
It seems to start with a fever, followed by a dry cough and then, after a week, leads to shortness of breath and some patients needing hospital treatment.
Around one-in-four cases are thought to be severe.
Notably, the infection rarely seems to cause a runny nose or sneezing.
The coronavirus family itself can cause symptoms ranging from a mild cold all the way through to death.
“When we see a new coronavirus, we want to know how severe are the symptoms. This is more than cold-like symptoms and that is a concern but it is not as severe as Sars,” says Prof Mark Woolhouse, from the University of Edinburgh.
The World Health Organization (WHO) says it is an emergency in China, but decided not to declare an international public health emergency – as it did with swine flu and Ebola.
How deadly is it?
More than 100 people are known to have died from the virus – but while the ratio of deaths to known cases appears low, the figures are unreliable.
It is far too simplistic to divide the number of deaths by the number of cases to calculate the death rate at this stage of the outbreak.
But the infection seems to take a while to kill, so more of those patients may yet die.
And it is unclear how many unreported cases there are.
Where has it come from?
New viruses are detected all the time.
They jump from one species, where they went unnoticed, into humans.
“If we think about outbreaks in the past, if it is a new coronavirus, it will have come from an animal reservoir,” says Prof Jonathan Ball, a virologist at the University of Nottingham.
Many of the early coronavirus cases were linked to the South China Seafood Wholesale Market, in Wuhan.
But the earliest documented case, which has been traced back to 1 December, had no connection to the market.
Sars started off in bats and then infected the civet cat, which in turn passed it on to humans.
And Middle East respiratory syndrome (Mers), which has killed 858 out of the 2,494 recorded cases since it emerged in 2012, regularly makes the jump from the dromedary camel.
Once the animal reservoir (where the virus normally camps out) is detected, then the problem becomes much easier to deal with.
While some sea-going mammals can carry coronaviruses (such as the Beluga whale), the South China Seafood Wholesale Market also has live wild animals, including chickens, bats, rabbits, snakes, which are more likely to be the source.
Researchers say the new virus is closely related to one found in Chinese horseshoe bats.
However, this does not mean wild bats are the source of the outbreak – they could have passed the virus onto another species sold at the market.
Prof Woolhouse says it is because of the size and density of the population and close contact with animals harbouring viruses.
“No-one is surprised the next outbreak is in China or that part of the world,” he says.
How easily does it spread between people?
At the beginning of the outbreak, the Chinese authorities said the virus was not spreading between people – but now, such cases have been identified.
Scientists have now revealed each infected person is passing the virus on to between 1.4 and 2.5 people.
This figure is called the virus’ basic reproduction number – anything higher than 1 means it’s self-sustaining.
We now know this is not a virus that will burn out on its own and disappear.
Only the decisions being made in China – including shutting down cities – can stop it spreading.
While those figures are early estimates, they put coronavirus in roughly the same league as Sars.
When are people infectious?
Chinese scientists say people are infectious even before their symptoms appear.
The time between infection and symptoms – known as the incubation period – lasts between one and 14 days.
Sars and Ebola are contagious only when symptoms appear. Such outbreaks are relatively easy to stop: identify and isolate people who are sick and monitor anyone they came into contact with.
Flu, however, is the most famous example of a virus that you spread before you even know you’re ill.
Prof Wendy Barclay from the department of infectious disease at Imperial College London said it was common for lung infections to spread without symptoms.
The virus is “carried into the air during normal breathing and talking by the infected person,” she explained.
“It would not be too surprising if the new coronavirus also does this.”
We are not at the stage where people are saying this could be a global pandemic like swine flu.
But the problems of stopping such “symptomless spreaders” will make the job of the Chinese authorities much harder.
What is not known is how infectious people are during the incubation period.
How fast is it spreading?
It might appear as though cases have soared. But this is somewhat misleading.
Many of these seeming new cases will have come to light as a result of China improving its ability to find infected people.
There is actually very little information on the “growth rate” of the outbreak.
But experts say the number of people becoming sick is likely to be far higher than the reported figures.
A report last week by the MRC Centre for Global Infectious Disease Analysis at Imperial College London said: “It is likely that the Wuhan outbreak of a novel coronavirus has caused substantially more cases of moderate or severe respiratory illness than currently reported.”
And over the weekend, researchers at Lancaster University estimated the number of cases suggesting 11,000 have been infected this year. If true, that would be more than Sars.
Could the virus mutate?
Yes, you would expect viruses to mutate and evolve all the time. But what this means is harder to tell.
China’s National Health Commission has warned the coronavirus’s transmission ability is getting stronger, but they were unclear on the risks posed by mutations of the virus.
This is something scientists will be watching closely.
How can the outbreak be stopped?
We now know the virus will not stop on its own; only the actions of the Chinese authorities can bring this epidemic to an end.
The only option is to prevent people who have become infected from spreading the virus to others.
- limiting people’s movement
- treating patients in isolation with healthcare workers wearing protective gear
A massive feat of detective work will also be needed to identify people whom patients have come into contact with to see if they have the virus.
Are there any vaccines or treatments?
However, the work to develop them is already under way. It is hoped that research into developing a vaccine for Mers, which is also a coronavirus, will make this an easier job.
And hospitals are testing anti-viral drugs to see if they have an impact.
A combination of two drugs – lopinavir and ritonavir – was successful in the Sars epidemic and is being tested in China during this outbreak.
Treatment at the moment relies on the basics.
Patients are kept in isolation so they do not spread the virus; breathing support is given to people with the worst lung disease; and doctors manage the other conditions the patients have.
How have Chinese authorities responded so far?
China has done something unprecedented anywhere in the world – by effectively putting Wuhan into quarantine.
Travel restrictions have also been imposed on a dozen other cities with 36 million people affected.
Some mass gatherings have been banned and tourists sites, including part of the Great Wall, have been closed.
And a ban on the sale of wildlife, a possible source of the infection, has been imposed.
Wuhan – the centre of the outbreak – is building two new hospitals with beds for a total of 2,300 people.
How is the world responding?
Most Asian countries have stepped up screenings of travellers from Wuhan and the WHO has warned hospitals worldwide a wider outbreak is possible.
Singapore and Hong Kong have been screening air passengers from Wuhan and authorities in the US and the UK have announced similar measures.
However, questions remain about the effectiveness of such measures.
If it takes up to two weeks for symptoms to appear, then someone could easily be halfway round the world and have passed through any screening checks before starting to feel ill.
How worried are the experts?
Dr Golding says: “At the moment, until we have more information, it’s really hard to know how worried we should be.
“Until we have confirmation of the source, that’s always going to make us uneasy.”
Prof Ball says: “We should be worried about any virus that explores humans for the first time, because it’s overcome the first major barrier.
“Once inside a [human] cell and replicating, it can start to generate mutations that could allow it to spread more efficiently and become more dangerous.
“You don’t want to give the virus the opportunity.”
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