The city of Hong Kong and its 7 million+ residents have reason to be extremely anxious about the nCoV outbreak that has already caused more than 1,000 deaths on the mainland. Back in 2003, SARS ripped through the densely populated largely autonomous city and killed some 300 people, nearly half the total death toll from the outbreak.

Professor Gabriel Leung, the chair of public health medicine at Hong Kong University, was one of the first officials anywhere in China to suggest that the government was hiding, or simply hadn’t yet confirmed, the true extend of the outbreak.

Though Beijing has been touting a ‘slowdown’ in the number of newly diagnosed cases, few believe that the outbreak has actually crested, even as a huge percentage of the population in the world’s largest country has spent the last week huddled inside.

Which is why we feel Leung’s latest warning is worth our attention, and yours.

Riffing off of comments from WHO Director-General Dr. Tedros Adhanom Ghebreyesus, who said yesterday that the we might be seeing only ‘the tip of the iceberg’ in terms of number of cases, Leung said the scientific community’s ‘overriding concern’ is figuring out the ‘size and shape’ of the iceberg.

Leung added that most experts now believe the virus has a transmission rate of – or r-sub-zero – of 2.5, meaning the average infected individual will transmit the virus to 2.5 others. This also translates to an “attack rate” of 60%-80%, the Guardian reports.

“Sixty per cent of the world’s population is an awfully big number,” Leung told the Guardian in London, en route to an expert meeting at the WHO in Geneva on Tuesday.

On Tuesday morning, Chinese health officials confirmed nearly 100 new deaths, bringing the toll to 1,013 as of late Monday.

If the virus continues to spread at this pace, even a relatively low fatality rate of 1% – which Leung believes is possible once milder, undetected cases are accounted for – could still lead to a massive death toll. Rough calculations indicate that, if two-thirds of the 7.7 billion people living on earth are infected, a 1% mortality rate would still lead to nearly 51 million deaths.

Once all of these other variables have been determined, Leung said he would tell the WHO that the main issues would be figuring out the scale of the worsening global epidemic, and learning whether China’s draconian measures have worked to help suppress the spread.

The upcoming meeting in Geneva, which Leung plans to attend, will bring together more than 400 researchers and national authorities. Some plan to participate by video conference from the mainland and Taiwan.

“With 99% of cases in China, this remains very much an emergency for that country, but one that holds a very grave threat for the rest of the world,” Dr. Tedros said last week.

One of the world’s leading experts on coronavirus epidemics, Leung played a major role in fighting the SARS outbreak and has worked closely with other leading scientists, including counterparts in the UK.

Does Leung really think the virus will infect 80% of the world’s population? Or even 60%? Maybe not. The virus has reportedly been mutating and changing as it has spread, and it’s still possible it could change in ways that inadvertently help humanity suppress it. For example, it could “attenuate its lethality,” as Leung put it.

Epidemiologists and modellers were trying to figure out what was likely to happen, said Leung. “Is 60-80% of the world’s population going to get infected? Maybe not. Maybe this will come in waves. Maybe the virus is going to attenuate its lethality because it certainly doesn’t help it if it kills everybody in its path, because it will get killed as well,” he said.

But if we don’t figure out exactly how bad the outbreak has already gotten, it will be much more difficult to stop it from arriving at the worst-case scenarios.