Nobody used to pay much mind to the coronaviruses. While the genus is home to two species responsible for the common cold, they haven't received the attention given to other cold-causing viruses because coronaviruses are difficult to grow in a lab environment. That all changed very quickly in 2003 when a new respiratory disease began killing doctors and nurses and showing the potential to spread at pandemic levels was identified as a previously unknown coronavirus. The infection was severe acute respiratory syndrome, or SARS, and it held the world's attention for just under a year before it disappeared in the summer of 2003.
The global public health response was a near-unparalleled success. Within weeks, control efforts led by the World Health Organization had identified a totally novel agent, devised a diagnostic test, and instituted plans for quarantine and isolation. It is undoubtedly a result of those efforts that the outbreak was contained before it could reach pandemic levels.
And while it is no longer topping watch lists, two questions persist: how did it get to humans and where did it come from? As Dr. Scott Dowell, head of the CDC's Global Disease Detection Program explains, "how it is that one of these animal pathogens acquires the ability to spread efficiently among humans is something that we don't do a very good job explaining or predicting."
Coronaviruses are known to mutate rapidly, so there may have been some biological basis to its sudden appearance and virulence, but it was still very much a surprise. Where it currently lies in wait is even more of an unknown. There is evidence the 2003 outbreak originated in a wildlife market in southern China, but the exact species of animal from which it came is still very much in contention.