An expert advisor to the World Health Organization (WHO) has described the coronavirus outbreak as potentially "thermonuclear pandemic level bad".
Dozens have been killed by the virus 2019-nCoV since it was first detected in early December, and as of Sunday morning (NZ time) there were 1441 confirmed cases.
But research into the disease - which at present has no vaccine, and might not for months - has doctors worried. Some have estimated for every person that gets infected, they will infect up to four more.
Others have come to figures of 3.8, 3.3, and 2.6, all above the WHO's estimate of between 1.4 and 2.5.
It's also been discovered people can carry the virus without showing symptoms, doctors calling them "cryptic cases of walking pneumonia".
Only 5.1 percent of all cases in Wuhan have been identified, researchers in the UK estimate, with perhaps more than 11,000 infected by January 21.
"HOLY MOTHER OF GOD," Dr Eric Feigl-Ding wrote on his Twitter account on hearing the news.
"It is thermonuclear pandemic level bad... I'm not exaggerating."
Dr Feigl-Ding is a health expert at Harvard University and has a dual doctorate in epidemiology and nutrition.
"Some folks think I'm trying to incite fear. I'm not trying - I'm a scientist. This [virus] is serious."
By February 4 - barely more than a week from now - scientists estimate between 132,000 and 273,000 will be infected.
China has moved to isolate cities with infections and stop travel, but scientists say this might be futile.
"Our model suggests that travel restrictions from and to Wuhan city are unlikely to be effective in halting transmission across China; with a 99 percent effective reduction in travel, the size of the epidemic outside of Wuhan may only be reduced by 24.9 percent," a paper uploaded to medRxiv reads.
"With these caveats in mind, our work suggests that a basic reproductive number for this 2019-nCoV outbreak is higher compared to other emergent coronaviruses, suggesting that containment or control of this pathogen may be substantially more difficult."
The UK scientists said it's still "early days", but it's looking like 2019-nCoV is spreading faster than SARS did in 2002/3. SARS went on to kill 774 people.
Dr Feigl-Ding said we're facing "possibly an unchecked pandemic that the world has not seen since the 1918 Spanish Influenza".
"Let's hope it doesn't reach that level but we now live in the modern world with faster [planes and trains] than 1918."
The Spanish Flu wasn't as contagious as 2019-nCoV appears to be, with patients only infecting about two others on average - and it killed possibly as many as 50 million people.
Dr Feigl-Ding called on WHO to declare an emergency "ASAP". WHO last week said it was "too soon" to declare an emergency. They're set to meet again this week to reevaluate that decision.