The World Health Organization (WHO) has previously said the "large acceleration" of cases in the United States meant it had the potential to become the new epicentre for the illness.

While WHO has strongly urged countries to test as much as possible, medical professionals have come under pressure due to a limit supply of tests and much-needed ventilators.

"Any scenario that is realistic will overwhelm the capacity of the healthcare system," said New York Governor Andrew Cuomo. New York has been hit the hardest of all of the states by the virus, with nearly 40,000 cases there alone.

"The number of ventilators we need is so astronomical - it's not like they have them sitting in the warehouse...There is no stockpile available."

Despite warnings, including from his own officials, that the country is not getting better anytime soon, US President Donald Trump has said he wants to reopen the country within a month. Like many countries, the US has extensive border measures and state lockdowns.

"We lose thousands and thousands of people a year to the flu. We don't turn the country off," Trump told Fox News on Tuesday (local time).

"We're opening up this incredible country, because we have to do that. I would love to have it open by Easter."

The Senate has also passed a bipartisan $2 trillion stimulus package to help businesses and millions of Americans hit by the economic fallout of the coronavirus pandemic.

Speaking to the G20, which was attended via audio-visual link by Trump and other world leaders, the Director-General of the WHO, Dr Tedros Adhanom Ghebreyesus called the pandemic the "defining health crisis of our time".

"We are at war with a virus that threatens to tear us apart - if we let it… This is a global crisis that requires a global response. Fight, unite, ignite."

Dr Ghebreyesus said the pandemic is now accelerating at an "exponential rate".

"Without aggressive action in all countries, millions could die."

A study released earlier this month from the Imperial College of London warned that without intensive actions to combat the virus more than a million Americans could die.

China saw an exponential rate of cases recorded there in January and February, but is now only reporting a small number of cases a day. It's believed most of those are imported cases, sparking worries of a second wave of the virus. However, in response China has closed its borders.

There have been concerns, though, that China may be downplaying the number of cases it is dealing with.

Largely due to its ageing population, Italy was also hard hit by the illness and continues to see hundreds of deaths every day from it. The European nation has the highest death toll of any country with at least 8000 killed.

There are more than 525,000 cases of the illness worldwide and there have been 23,000 deaths. In New Zealand, there are 283 cases, but no deaths. New Zealand has gone into a state of nationwide lockdown to avoid spreading the virus. That requires people to stay indoors and for non-essential businesses to shut.

While countries, including Aotearoa, have said their lockdown protocols will be in place for several weeks, subject to review and extension, experts suggest they would need to be in place for a longer period of time, possibly until a vaccine is created, to properly stop the virus' spread. That vaccine could be up to 18 months away.

What we know about coronavirus

The World Health Organization was first notified of cases of the virus SARS-CoV-2 (Severe Acute Respiratory Syndrome Coronavirus 2) in Wuhan, China on December 31. It was identified as a coronavirus on January 7 and can spread via human-to-human transmission. It causes the coronavirus COVID-19 illness.

The virus is primarily spread through droplets in the air after someone sneezes or coughs, however, it can also be contracted by touching surfaces where the illness is present. The length of time the virus stays alive on surfaces isn't fully understood, but some viruses can remain active for days.

"Common signs of infection include respiratory symptoms, 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 WHO says.

"Standard recommendations to prevent infection spread include regular hand washing, covering mouth and nose when coughing and sneezing, thoroughly cooking meat and eggs. Avoid close contact with anyone showing symptoms of respiratory illness such as coughing and sneezing."

There is currently no vaccine for the sickness.

How can I protect myself?

avoid touching the mouth, nose and eyes with unwashed hands

washing your hands before eating

carrying a hand sanitiser at all times

being particularly mindful of touching your face after using public transport or going to the airport

carry tissues at all times to cover the mouth and nose when coughing or sneezing (then dispose of it)

not eating shared or communal food

avoiding shaking hands, kissing cheeks

regularly cleaning and sanitise commonly used surfaces and items, such as phones and keys

avoiding close contact with people suffering from or showing symptoms of acute respiratory infection

seeking medical attention if you feel unwell.

An explainer on protecting yourself from coronavirus can be found here. Full information can also be found at Covid19.govt.nz

The Ministry of Health is reminding the public to get in touch with Healthline on 0800 358 5453 if they have symptoms or concerns.