Top health experts say coronavirus could infect 40-60 per cent of the global population, but only if the virus is not controlled.
Michael Baker, University of Otago professor of public health, told the Herald that was only a forecast number.
"If the pandemic is not controlled or contained in a country, the epidemiology information suggests that it would infect 40–60 per cent of the world's population.
"In most countries of the world now, people are changing behaviours to suppress transmission of the virus, so the reality is that it will be much less than the global estimate, which is based on no controlled efforts."
Most countries were containing the spread of the virus, so only a "tiny proportion" of the population is affected, Baker said.
"China, after all, has contained it very effectively - it's only a tiny portion of their 1.4 billion people who are infected."
"Other countries are also achieving successful containment: That's Singapore, Hong Kong and Taiwan and they've got really very few people infected compared with what would've happened if they weren't containing it."
Five people have been confirmed to have Covid-19 in New Zealand, all of whom are being cared for at home. No new cases have been announced in the past four days.
Dr Samantha Murton, President of the Royal New Zealand College of General Practitioners, told Mediaworks some estimates showed half of all New Zealanders could contract the illness.
"At the moment, what we are sort of looking at is that at least 50 per cent would probably be infected," she said.
"What the measures we are putting in place are trying to do is reduce the extreme nature of that, so we can get a reduced number."
However, infectious diseases expert Dr Siouxsie Wiles said that was the worst-case scenario.
"From the data we have so far, we would expect 80 per cent of those people have mild to moderate symptoms which may mean they're stuck at home for a couple of weeks."
The other 20 per cent of those infected would face a two-to-six-week hospital stay.
Of those, a small number would die, Wiles said.
"So, that's if we did nothing. But it's very clear that we're not going to do nothing."
How many people would be infected depended on how people behaved, she said.
"Everything is about, how do we stop transmission and how do we protect the most vulnerable in our communities who would be the most impacted?"
Baker said countries that don't contain the rapid spread of the virus will see a large chunk of its people infected.
"At the moment, fortunately, New Zealand is in containment mode," he said.
While the broader measures were working, now wasn't the time to be complacent, Baker said.
"We just have to expect that there will be huge stress on the system in the weeks and months ahead so we should be really ramping up our capabilities."
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The world is going to split into two groups of countries over the next weeks and months: Those that have contained Covid-19 and those that haven't, Baker said.
"We obviously want to be amongst the countries that have contained it.
"I think New Zealand can really do it, but we have to be using every day that we can to plan for it and ramp up in all these areas."
Director-General of Health Ashley Bloomfield earlier urged anyone feeling unwell to stay home.
Everyone - including himself - was guilty at times of going to work with a runny nose and a sore throat, or sending their slightly sick kid to school. Now was the time to be much stricter, he said.
"Please stay home if you are unwell".
This was particularly important with large public events such as Auckland's Pasifika festival and the memorial service for the Christchurch terror attacks this weekend.