A new coronavirus that emerged in China has killed more than 1,000 people, infected at least 45,000 and spread to two dozen countries, prompting the United Nations health agency to declare the pathogen the "public enemy number one".

All but two deaths and 99 percent of infections have been reported in mainland China, but Tedros Adhanom Ghebreyesus, director general of the World Health Organization (WHO), said on Tuesday that the outbreak "holds a very grave threat" to the rest of the world, especially countries with weaker health systems.

Here's what you need to know about how the virus spreads and how you can protect yourself.

How is the new coronavirus spreading?

The new coronavirus, officially named COVID-19, is thought to have jumped to a human from a yet-to-be-identified animal host. Similar to other coronaviruses - such as those that cause the common cold and the severe acute respiratory syndrome (SARS) - the new virus causes respiratory disease.

It can be transmitted from person to person and is spread primarily through contact with an infected person via respiratory droplets generated when a person coughs or sneezes.

It can also be transmitted through droplets of saliva and discharge from the nose.

People can also become infected via contaminated surfaces. But the virus weakens significantly when exposed to air, with preliminary information suggesting it may survive a few hours or more on surfaces.

It takes about five to six days for an infected person to show symptoms, but the incubation period could be as long as two weeks.

How contagious is the new virus?

Based on current estimates, scientists say the new virus has a reproductive value of between 2.24 to 3.58, meaning an infected person could pass on the pathogen to between two and four people. In order to contain the outbreak, that value has to be reduced to below 1.

In order to limit the spread of the virus, China has placed the city of Wuhan, the epicentre of the virus, under a virtual lockdown. It has done the same with surrounding cities, too. The country's national health agency said it is tracking nearly half a million people who have had close contact with infected people and is medically observing 185,000 others.

200205205234883

The WHO says it is mostly infected people who have symptoms who are causing the spread of the virus, but some reports indicate that some of those infected may pass on the virus before showing significant symptoms.

Some experts say this could complicate efforts to contain the spread of the disease.

"One of the key points [in containing COVID-19] is going to be whether we can use this approach of isolating and tracing contact cases effectively. That works very well for SARS because you didn't have a lot of transmission occurring without symptoms," said Adam Kuscharski, an epidemiologist at the London School of Hygiene and Tropical Medicine.

"We've still to work out how much of transmission is [asymptomatic] and if it's enough to make something like contact tracing very difficult to be successful."

The vast majority of cases are mild with the most common symptoms being a dry cough, fever and fatigue. But in more severe cases, the disease can lead to pneumonia and more rarely, multiple organ failure and death.

"[The virus] looks like other infections and of course that's dangerous for a new emerging pathogen because basically you can't tell straight away what you're dealing with," David Carrington, virologist at St Georges University Hospitals in London, told Al Jazeera.

"The majority of individuals don't have that complicated pneumonic picture, they have more respiratory features that in themselves will not be able to be identified quickly if you're looking for this virus."

Read more on what the coronavirus does to your body if you get infected here.

How long will the outbreak last?

On Wednesday, China's national health agency reported a decline in the number of new cases for a second straight day in Hubei Province, of which Wuhan is the capital, raising hopes the outbreak could peak later in February. Outside Hubei, the number of new patients has been declining for the past week.

Zhong Nanshan, China's top medical expert on the coronavirus, told Reuters news agency on Tuesday that he expected the epidemic to end by April.

But some experts warn that estimate may be too optimistic.

"There is certainly some evidence that the Chinese part of the outbreak both within Hubei Province and within other areas within China might be on the decline," said Paul Hunter, professor at Norwich Medical School.

He added: "The SARS outbreak burned itself out in eight months, but this seems to be substantially more infectious than SARS, and despite the encouraging trends in the last four or five days, I personally would be surprised if it does [end] in the current year."

The SARS outbreak of 2002 and 2003 infected some 8,000 people and killed nearly 800.

How can you protect yourself?

Scientists around the world are racing to develop a vaccine, but the WHO says an effective candidate is at least 18 months away. Researchers are also examining the use of antivirals as a treatment.

"The development of vaccines and therapeutics is one important part of the research agenda. But it's not only one part," said Tedros, the WHO chief. "They will take time to develop but in the meantime, we're not defenceless. There are many basic public health interventions that are available to us now and which can prevent infections now."

200206120657215

The most effective way to limit infection, according to WHO, is practicing good hygiene, including washing your hands frequently with soap and water or an alcohol-based rub, and covering your mouth and nose when coughing or sneezing.

WHO also suggests avoiding close contact with sick people, and seeking medical care early if you have fever, cough or difficulty breathing.

Do face masks help?

Fear over the virus outbreak has triggered a global rush for protective masks, with shops and pharmacies in many countries selling out of face masks.

WHO has called for "rational use" of medical masks with Michael Ryan, the agency's executive director for emergencies, warning: "Masks don't necessarily protect you but they do, if you have the disease, stop you from giving it to anybody else."

WHO said the stockpiling of surgical face masks and protective suits is depleting stocks required for front-line healthcare workers and those who are already sick, and recommends that healthy people only use protective masks when taking care of a person who is suspected of carrying the virus.

Those using masks should follow safety guidelines to ensure their efficacy, by disinfecting hands before and after use, and making sure there are no gaps between the face and mask.

Single-use masks should be disposed of once they are damp. They should be placed in a sealed bin, and not reused.