On January 30, the World Health Organization declared the novel coronavirus — then unnamed — to be a "Public Health Emergency of International Concern." The virus, first reported in China in late 2019, had started to spread beyond its borders, causing 98 cases in 18 countries in addition to some 7,700 cases in China at the time.
Six months later, the tiny coronavirus has spread around the world, infecting more than 16 million people worldwide and killing more than 650,000. It is one of the leading causes of death in the U.S. in 2020.
"This is the sixth time a global health emergency has been declared under the International Health Regulations, but it is easily the most severe," said Tedros Adhanom Ghebreyesus, WHO's director-general, on Monday.
What was it about this coronavirus — later named SARS-CoV-2 — that made it the one to spark a global pandemic?
Virologists point to several key traits that this virus possesses. Any one of them might be problematic. When combined in one microscopic virus, the result is what coronavirus researcher Andrea Pruijssers of Vanderbilt University calls a "perfect storm" — a one-in-a-million virus capable of triggering a worldwide health crisis.
It's a super-fast spreader ...
One of the novel coronavirus's biggest advantages is how easily it spreads from human to human, says Dr. Megan Freeman, a virologist at Children's Hospital of Pittsburgh, who conducted her doctoral research on coronaviruses.
The coronavirus causes COVID-19, a respiratory disease that infects the sinuses, throat, lungs — all parts of the body involved with breathing. As a result, the virus can be readily passed onward through breath and spittle expelled from the nose and mouth. Unlike Ebola, where direct contact with blood and other bodily fluids is the main route of infection, you don't have to touch someone to be infected with SARS-CoV-2 — all it takes is getting close enough to an infected person and breathing in respiratory droplets they exhale.
And because it's transmitted rapidly through the respiratory route, "it's a virus that [also] has the capacity to spread across the globe fairly easily," Pruijssers says. All it takes to introduce the illness to a new continent is a single person who travels there while infectious.
... but not so fast that it'll knock itself out
When a virus spreads too quickly, enough people in a community may catch it to create "herd immunity." With fewer people to infect, the virus's rapid spread can ensure its own demise, says Malik Peiris, a virologist at the University of Hong Kong. "It's a balance," he says, pointing out that other viruses such as dengue fever and chikungunya have surged and then died down in certain locations.
It's transmittable even with no symptoms
Even before symptoms develop, infected people can spread this virus by speaking, singing, coughing and breathing out virus-laden droplets in close proximity to others. "For SARS-CoV-2, a lot of the transmission is from asymptomatic, [presymptomatic] or mildly symptomatic people," Pruijssers says.
By contrast, SARS-CoV-1, a related coronavirus that caused an epidemic in Asia in 2003, was most infectious when people were symptomatic. So as soon as someone showed symptoms, they were quarantined — which effectively stopped that virus from transmitting, Peiris says. The SARS epidemic officially ended in 2004 after sickening 8,098 people; there have been no known cases reported since.
The severity of symptoms puts a strain on health systems
Even though some people who are infected have no symptoms or mild symptoms, the novel coronavirus can inflict serious damage. "This coronavirus has the capacity to cause really debilitating respiratory disease and even death" for a higher proportion of infected people compared with, say, the flu, Freeman says.
Because COVID-19 can make people sick enough to require hospitalization, high rates of spread have strained hospital systems, making it difficult to provide optimal care for patients, as is happening in California and Texas. When hospitals run low on staff and supplies, the result can be care rationing and excess deaths. Since its emergence, in late 2019 in China, the novel coronavirus has killed more than 600,000 globally.
Then there's the pet theory ...
Not only did the novel coronavirus come from animals, it also appears to have the ability to jump from humans to animals, including their pets — and possibly back again.
The virus likely originated in bats and spilled over to humans because of some unlucky coincidence, where a person was "in the wrong place at the wrong time" and came in contact with a bat or an intermediary animal that happened to be infected with this particular virus, says Carlos Zambrana-Torrelio, an ecologist with the nonprofit organization EcoHealth Alliance.
Now, researchers have found humans have occasionally infected their pet dogs and cats as well as lions and tigers at the Bronx Zoo. There's no evidence yet of dogs and cats passing it to people, but sick minks on Dutch fur farms are thought to have given the coronavirus back to humans.
This could mean that if the virus starts circulating regularly among animals that we handle or live with, it may be really hard to get rid of it, Freeman says. "[If] there's an animal reservoir, there's always that possibility that the virus could come back in a spillover event," she says. In other words, a community could be virus-free only to have it reintroduced by a visiting animal.
... and this virus has the element of surprise
The world has never dealt with a pandemic caused by a highly dangerous coronavirus before. This means everyone in the world is likely susceptible to it and also that, in the beginning "we knew nothing about it — it was a brand new virus," Pruijssers says. And that lack of knowledge about treatments and control has contributed to the virus's ability to spread.
Unlike flu, which has been known to researchers for centuries, this novel coronavirus has required researchers to figure out everything from scratch — how it spreads, who's most likely to get sick from it and how to combat it with drugs and vaccines.
There's still a lot we don't know, Pruijssers says, and we're learning fast. But not fast enough to have stopped this pandemic from happening.
ARI SHAPIRO, HOST:
Many viruses have the potential to cause pandemics, but it is the rare virus that actually succeeds. NPR's Pien Huang looks into the superpower combination of traits that have helped the coronavirus go global.
PIEN HUANG, BYLINE: Tedros Adhanom Ghebreyesus, director-general of the World Health Organization, says the coronavirus is a once-in-a-century virus.
TEDROS ADHANOM GHEBREYESUS: It has very peculiar features. And the world has never seen anything like this for several decades.
HUANG: All the way back to the 1918 flu pandemic. Carlos Zambrana-Torrelio agrees. He's an ecologist with the nonprofit EcoHealth Alliance, and he says there are hundreds of coronaviruses in bats alone that could theoretically infect people. He also estimates that there are 1.7 million viruses living in other mammals and birds.
CARLOS ZAMBRANA-TORRELIO: A fraction of those are pathogenic, and just a few of those will become pandemic.
HUANG: Most of these viruses don't have the right combination to be completely explosive. Dr. Megan Freeman, a virologist at Children's Hospital of Pittsburgh, says the coronavirus combines a few key traits that make it really difficult to control. Some of these characteristics are well known by now.
MEGAN FREEMAN: So I think transmissibility from human to human is one of its biggest superpowers.
HUANG: Freeman says it's highly contagious, and you don't have to touch someone to get it. All it takes is getting close enough to someone who is infected and breathing in their respiratory droplets. One infected person can get on a plane and spread the virus to a new continent. Another trait, says Andrea Pruijssers, a virologist at Vanderbilt University, is when it's contagious.
ANDREA PRUIJSSERS: For SARS-CoV-2, a lot of the transmission is from asymptomatic or mildly symptomatic people.
HUANG: That makes it really hard to stop people from passing the virus on. You basically have to assume that anyone you come in contact with could be infected at any given time without knowing it. That's why mask-wearing is required in many public spaces. Pruijssers says another trait we're starting to learn about is the virus's ability to jump from people to different kinds of animals.
PRUIJSSERS: We know that it can affect cats and ferrets and minks and all of these animals that are living in close proximity to humans.
HUANG: Pruijssers says this suggests that if the virus starts circulating regularly among animals that we handle or live with it, it might be really hard to ever get rid of. A community could be virus-free only to have it reintroduced by a visiting cat. So the coronavirus is highly contagious, it spreads without symptoms, and it jumps with ease between humans and animals. Lots of viruses have some of these traits, but altogether they've created a perfect-storm virus that's causing a once-in-a-century pandemic.
PRUIJSSERS: And, of course, we know nothing about it. It's a brand-new virus.
HUANG: Pruijssers says it means that everyone is susceptible to it. And researchers have had to figure out everything from scratch - how it spreads, who's more likely to get sick from it and how to combat it with drugs and vaccines. She says there's a lot we still don't know, but we are learning fast. Pien Huang, NPR News.
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