Coronavirus: How worried should we be? By James Gallagher Health and science correspondent A virus - previously unknown to science - is causing severe lung disease in China and has also been detected in other countries. Fifty-six people are known to have died from the virus, which appeared in the Chinese city of Wuhan in December. There are already more than 2,000 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? Coronavirus: Your questions answered Wuhan: The London-sized city where the virus began China coronavirus: What we know so far 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. 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? Fifty-six people are known to have died from the virus - but while the ratio of deaths to known cases appears low, the figures are unreliable. 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. 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. Which animal? Once the animal reservoir (where the virus normally camps out) is detected, then the problem becomes much easier to deal with. The coronavirus cases have been linked to the South China Seafood Wholesale Market, in Wuhan. But while some sea-going mammals can carry coronaviruses (such as the Beluga whale), the 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. Why China? 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 have confirmed 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 infectious 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, from 40 to more than 2,000 in just over a week. But this is 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.