A newly identified virus that emerged late last year in the central Chinese city of Wuhan, causing an explosive outbreak of sometimes fatal pneumonia, has become a global health emergency. The culprit is a so-called coronavirus that causes a respiratory illness dubbed Covid-19. In the first two months of the outbreak, it killed more people than died in nine months from a related virus responsible for the 2002-2003 epidemic of severe acute respiratory syndrome, or SARS.

1. What makes this virus so bad?

It has been described as “insidious” because a large portion of infected people are well enough to go about their daily business, potentially spreading it to others. As of mid-February the apparent mortality rate was about 2%, lower than the 9.5% rate for SARS. But such numbers are unreliable in the early stages of an outbreak. Some disease modeling experts project as many as hundreds of thousands of people are actually infected, most of whom won’t even know they have it. One study published Feb. 10 estimated a mortality rate of 1% once all cases, including those with no or only mild symptoms, are counted. A revision in the method of testing for the virus produced a jump in the number of detected cases around mid-February.

2. What does the virus do?

Infections appear to cause a mild illness in children, adolescents and younger adults in most cases, and potentially more severe disease in older people. Frequently reported early signs are fever, dry cough and tiredness -- but not the sneezing and runny nose typical of a cold. In severe cases, studies suggest the virus invades cells in the lower respiratory tract and migrates to the lungs, causing difficulty breathing and the inflammation and congestion associated with pneumonia. In an early study, more than a quarter of hospitalized patients developed a complication known as acute respiratory distress syndrome. Many of the fatalities have been in patients with underlying illnesses such as cardiovascular disease. A Chinese study of 72,000 cases found the majority of deaths occurred in patients over 60 years old. Of confirmed cases, 81% were mild, 14% were severe and 4.7% critical.

3. How does this compare with other outbreaks?

The virus that causes Covid-19 is genetically similar to the SARS virus but appears milder in terms of illness and case fatality rate. Another related virus, known as MERS-CoV, that’s been spreading since 2012 has led to death in 34% of the 2,499 cases recorded. By contrast, an estimated 50 million people died in the 1918 influenza pandemic that had a case-fatality ratio of less than 5%, but infected up to a third of the world’s population.

4. How do people contract it?

Most likely by coming into contact with virus-containing droplets that are emitted when an infected person breathes, talks, coughs or sneezes. These can be transferred directly to someone else in close proximity or via hands and surfaces. How long it survives on surfaces is still not known, but preliminary studies suggest coronaviruses may remain infectious from a few hours to a few days. Simple disinfectants kill the virus. There’s a theoretical risk the virus can spread through feces or farther through the air in tiny particles known as aerosols. People who are still incubating the virus and show no symptoms may spread it.

5. How contagious is it?

Epidemiologists try to gauge contagiousness by estimating the number of additional people a person who is infected is likely to infect. That measurement, called a basic reproduction number, is one indicator of how difficult an epidemic is to control. The World Health Organization gives a preliminary range of 1.4 to 2.5 for Covid-19. At the high end, that makes it similar to SARS and more infectious than flu. A group from the Chinese Academy of Sciences in Beijing estimated the number to be 4.08.

7. What’s a coronavirus?

Coronaviruses are named for their crown-like shape. There’s a large family of them, responsible for diseases that range in severity from the common cold to MERS. Some transmit easily from person to person, while others do not. The WHO says that new strains emerge periodically around the globe, and several known versions are circulating in animals and haven’t infected humans.

8. Where did it come from?

The virus emerged in early December in Wuhan, an industrial city of 11 million and capital of Hubei province. Early attention focused on a seafood market where live animals were also sold, but about a third of the first 41 cases were found to have no link to it. The viral genome is even more closely related to several coronaviruses found in bats than it is to the SARS coronavirus. Diseases transmissible from animals to humans, sometimes referred to as “zoonoses,” comprise a large percentage of all newly identified infectious diseases.

9. How alarming is a new virus?

There is always concern when a new pathogen emerges in people because they typically lack immunity to it and there usually aren’t specific treatments or vaccines available. Novel coronaviruses, those not seen in humans before, represent a particular concern because they have been known to spark complicated outbreaks that have sickened thousands of people, like SARS did as it swept across the globe from southern China.

10. How worried should I be?

Unlike SARS, relatively few cases have been reported outside mainland China. But in Hubei, thousands of people have been hospitalized in isolation wards and hundreds are being treated in intensive care units, creating what officials there have described as an “extreme, severe and abrupt public health crisis.” A Wuhan hospital saw coronavirus infections spread rapidly among health-care staff and other patients, according to a study published in the Journal of the American Medical Association.

11. What are authorities doing?

China’s government imposed a quarantine on Wuhan and more than a dozen other cities in the region, a travel ban covering in excess of 50 million people. New hospitals were built from the ground up in days, and the production of medical equipment ramped up. The WHO has declared the outbreak a global health emergency, a designation that can help mobilize international responses. Anyone caught spreading misinformation was subject to criminal charges in some Asian countries. Hong Kong, the international financial center that functions with some autonomy from China, announced its own travel restrictions from the mainland. The U.S., Australia, India and other countries are denying or restricting entry for non-citizens arriving from China in an attempt to stop the virus’s spread, although the WHO has called such measures unnecessary.

12. How are the authorities faring?

Early praise for China’s response has ebbed. The country did not immediately release genetic information about the virus and has struggled to explain changes in the way it counts new cases. In a nation where the internet is heavily censored, there was a rare outpouring of social media fury over the death from the virus of a Chinese doctor who had waved an early red flag about the outbreak but was silenced by police. The top officials in Wuhan and Hubei were later removed from their posts. China’s strict quarantine likely bought the rest of the world two to three weeks to prepare for the virus, according to the WHO.

--With assistance from Dong Lyu, Hannah Dormido, James Paton, Grant Clark, Todd White and Michelle Fay Cortez.

To contact the reporters on this story: Jason Gale in Melbourne at j.gale@bloomberg.net;John Lauerman in London at jlauerman@bloomberg.net

To contact the editors responsible for this story: Michael Patterson at mpatterson10@bloomberg.net, Lisa Beyer, Paul Geitner

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