The World Health Organization has officially declared the coronavirus outbreak a pandemic. Tedros Adhanom Ghebreyesus, WHO director-general, made the announcement at a press briefing on Wednesday. Since first being detected in the central Chinese city of Wuhan in Dec. 2019, the virus has spread to every continent except Antarctica and infected more than 121,000 people, causing 4,300 deaths. 

The WHO has, in the past, defined a pandemic as "the worldwide spread of a new disease." The coronavirus, which causes the disease COVID-19, spread to over 110 countries within three months. On Jan. 30, the WHO announced the coronavirus outbreak was a "public health emergency of international concern" around the time almost 8,000 cases had been confirmed across 18 countries and the death toll was approaching 200. 

The WHO was reticent to call the COVID-19 outbreak a pandemic in an attempt to prevent panic and suggested containment measures were helping control the spread, but experts suggested the conditions were met in late February as case numbers in Italy, South Korea and Iran began to rapidly climb. On March 9, Tedros announced "the threat of a pandemic has become very real" but reiterated the spread of disease was still under control. 

The formal declaration of a pandemic doesn't mean it's time to panic. Rather, it allows health authorities around the world to begin planning for a new phase in battling the disease, outlining strategies to mitigate further spread and help the public understand the signs and symptoms of COVID-19 infection.  

WHO last declared a pandemic on June 11, 2009, when widespread transmission of the influenza virus H1N1 -- or "swine flu" -- rapidly spread across the globe. In that instance, it took the WHO a little under two months to move from a public health emergency notification to "pandemic." At the time, more than 70 countries had reported cases of the disease.