The epidemic of coronavirus worldwide has surpassed 100,000 cases globally in more than 100 countries.

Canada confirmed its first presumed case on Jan. 25, followed by more in Ontario, British Columbia, Quebec and Alberta.

The World Health Organization (WHO) declared the outbreak a public health emergency of international concern on Jan. 30 based on the potential for the virus to spread to countries with weaker health systems.

Canadian public health officials stress that the risk of contracting the illness in this country remains low. They've asked the general public to be vigilant and for health-care workers to prepare to care for infected patients.

Here is what we know — and what we don't know — about the virus.

People stand behind barriers at Saint Peter's Square in Vatican City. The square and main basilica were closed to tourists from Tuesday until April 3 as part of control measures against the spread of the COVID-19 in Vatican City, Italy. (Elisabetta A. Villa/Getty)

1. What is coronavirus?

Coronaviruses are a large family of viruses. They cause a range of illness ranging from the common cold to more severe diseases — such as Middle East Respiratory Syndrome (MERS-CoV) and Severe Acute Respiratory Syndrome (SARS-CoV).

SARS was the coronavirus that originated in China in late 2002, and which eventually killed 44 people in Canada and infected more than 400 before the outbreak in China was declared by the WHO to be "under control" on Apr. 28, 2004.

2. Where did this new coronavirus come from?

The WHO's China office was first informed of cases of pneumonia with an unknown cause on Dec. 31, 2019. The initial cases were all detected in Wuhan City.

A new coronavirus was identified as the probable cause by Chinese authorities Jan. 7.

The virus that causes the illness is now known as SARS-CoV-2​​​​.

The WHO reported the evidence was "highly suggestive" that the source was a seafood market that also sells live poultry and meat from exotic animals in Wuhan.

3. How is it transmitted?

Coronaviruses are zoonotic, meaning they originally pass from animals to humans. But some, like this one, can also pass directly between humans.

Chinese scientists confirmed there has been human-to-human transmission of the virus among close contacts such as family members. It's unclear how easily the virus is transmitted between people.

According to the Centers for Disease Control in the U.S., coronaviruses are most commonly spread by coughing or sneezing; close personal contact, such as shaking hands; or touching an object or surface with the virus on it and then touching your mouth, eyes or nose.

A man wears a mask aboard the Grand Princess cruise ship off the coast of San Francisco on March 8. (Noah Berger/The Associated Press)

4. What are the symptoms?

The initial symptoms of the illness, called COVID-19, are mainly fever and cough, with a few reports of people having difficulty breathing, and chest X-rays showing signs of pneumonia in both lungs.

According to the WHO, signs of infection can include respiratory complaints, fever, cough, shortness of breath and breathing difficulties. In more severe cases, infection can cause pneumonia, severe acute respiratory syndrome, kidney failure and even death.

The only way to confirm infection is with a lab test.

5. What should Canadians do?

Major problems with COVID-19 outbreaks have occurred on cruise ships since early February.

On March 9, Canada's Chief Public Health Officer Dr. Theresa Tam cautioned against travelling on cruise ships.

"I've asked Canadians to think twice about being on cruise ships. Today, the Public Health Agency of Canada is recommending that Canadians avoid all cruise ship travel due to COVID-19," Tam said.

"The risk to the general population remains low, but this could change rapidly. We are most concerned for Canada's vulnerable populations."

A woman in protective gear rides a bicycle past a mural in Wuhan, the epicentre of China's coronavirus outbreak, on Feb. 28. (Reuters)

Foreign Affairs Minister François-Philippe Champagne urges Canadians to keep track of Global Affairs Canada's travel advisories, calling the current outbreak situation "very fluid."

The department has updated its travel advisory for China, urging Canadians to take precautions such as avoiding large crowds and high-risk areas including farms and slaughterhouses, and avoiding contact with anyone who is exhibiting symptoms of a coronavirus, such as fever, cough or difficulty breathing.

The Public Health Agency of Canada has measures at 10 airports across the country to help identify any travellers returning to the country who may be ill and to raise awareness among travellers about what to do should they become sick.

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If travellers develop flu-like symptoms, they're advised to call ahead to health-care professionals.

The agency said it is important for all travellers to monitor their health for fever, cough and difficulty breathing for 14 days when they return to Canada.

Banners and information booths with staff from the Public Health Agency are also in place at international airports in Vancouver, Toronto-Pearson and Montréal, where all international travellers need to respond to a screening question on an electronic kiosk. The question is available in 15 languages.

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6. What we don't know yet

COVID-19 has created alarm because there are a number of unknowns surrounding it.

Since the virus is new to humans, the public health community believes that no one has existing immunity.

Early on in an outbreak, not everyone who is infected seeks or needs medical attention so officials won't have a clear picture of mild illness.

Not knowing the full spectrum makes it hard to estimate the fatality rate. But public health experts say early research suggests that the novel coronavirus is less deadly than other infectious diseases.

Doctors predict cases will continue to multiply, although the jump in numbers is also attributable in part to increased monitoring.

Of 44,000 cases in China, about four out of five were considered mild and didn't lead to pneumonia, Chinese researchers said.

While sweeping lockdowns and other measures are typical of China's government, large-scale quarantines are rare around the world — even in deadly epidemics — because of concerns about infringing on people's liberties. The effectiveness of such measures is also unclear.

There is no specific treatment or vaccine yet for the virus.

Notable quotes:

The main reason for this declaration is not because of what is happening in China but because of what is happening in other countries.

Our greatest concern is the potential for this virus to spread to countries with weaker health systems which are ill-prepared to deal with it.

- WHO Director-General Tedros Adhanom Ghebreyesus

.<a href="https://twitter.com/WHO?ref_src=twsrc%5Etfw">@WHO</a> doesn’t recommend limiting trade & movement.<br>Travel restrictions can cause more harm than good by hindering info-sharing & medical supply chains & harming economies. We urge countries & companies to make evidence-based, consistent decisions. <a href="https://t.co/ksxOV6sbDN">https://t.co/ksxOV6sbDN</a> —@DrTedros

Globally, we will not be able to contain the spread of this virus. We can slow it down, but we can't stop it.

- Dr. Allison McGeer, infectious disease consultant, Mount Sinai Hospital in Toronto, who worked on the SARS epidemic in 2003.

We are on highest alert for coronavirus spread. But this isn't meant to alarm or scare people. It's to get governments to understand that they need to get ready. You have a duty to your citizens and to the world to be ready.

- Dr. Michael Ryan, director of the WHO Health Emergencies Program