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 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 

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 

"This is the sixth time a global health emergency has been declared under the 

, but it is easily the most severe," said Tedros Adhanom Ghebreyesus, WHO's director-general, 

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 

 of Vanderbilt University calls a "perfect storm" — a one-in-a-million virus capable of triggering a worldwide health crisis.

One of the novel coronavirus's biggest advantages is how easily it spreads from human to human, says 

, 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 

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 "

." With fewer people to infect, the virus's rapid spread can ensure its own demise, says 

, a virologist at the University of Hong Kong. "It's a balance," he says, pointing out that other viruses such as 

 have surged and then died down in certain locations.

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 

 or mildly symptomatic people," Pruijssers says. 

SARS-CoV-1, a related coronavirus that caused an 

 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 

; 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 

. When hospitals run low on staff and supplies, the result can be 

Since its emergence, in late 2019 in China, the novel coronavirus has killed 

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. 

 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 

, an ecologist with the nonprofit organization EcoHealth Alliance.

Now, researchers have found humans have occasionally infected their pet 

 at the Bronx Zoo. There's no evidence yet of dogs and cats passing it to people, but sick 

 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.

 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.


Health

COVID-19

Pien Huang

Author

Many viruses have pandemic potential but never reach the tipping point. So what made this one capable of wreaking global havoc?

Why the novel coronavirus has the power to launch a pandemic

Go Deeper.

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