The 2019 novel coronavirus (2019-nCoV) behind the ongoing outbreak—which the World Health Organization has declared an international public health emergency—was named after the family of viruses it belongs to. The term “coronavirus” may have initially been unfamiliar to many, but most everyone has encountered milder forms of such viruses, of which four strains cause about a fifth of common cold cases. Other types cause diseases that are endemic in certain animal populations. But until less than two decades ago, all known human varieties caused illness so mild that coronavirus research was something of a backwater.
That all changed in 2003, when the pathogen behind the SARS (severe acute respiratory syndrome) outbreak in China was identified as a coronavirus. “Everybody in the field was shocked,” says microbiologist Susan Weiss of the University of Pennsylvania. “People started really caring about this group of viruses.” That outbreak is believed to have started when a coronavirus jumped from animals—most likely civet cats—to humans, resulting in a type of disease called a zoonosis. These viruses’ propensity for such jumps was underlined in 2012, when another virus jumped from camels to humans, causing MERS (Middle East respiratory syndrome). That illness has killed 858 people to date, primarily in Saudi Arabia, representing approximately 34 percent of those infected.
SARS, MERS and the new coronavirus almost certainly all originated in bats. The most recent analysis of the 2019-nCoV genome found it shares 96 percent of its RNA with a coronavirus previously identified in a specific bat species in China. “These viruses have been floating around in bats for a long time” without sickening the animals, says microbiologist Stanley Perlman of the University of Iowa. But there were no bats being sold at the animal market in Wuhan, China, where the current outbreak is thought to have begun, suggesting an intermediate host species was likely involved. This situation seems to be a common feature of these outbreaks. Such hosts may increase the viruses’ genetic diversity by facilitating more or different mutations.
But what is a coronavirus? What determines whether, when and how it jumps to humans and how infectious it will be? And what makes the difference between a case of the sniffles and a deadly disease? In the years since these viruses first emerged as a severe global health threat, researchers have been studying their molecular biology in an effort to answer such questions.