West African Ebola epidemic

In March 2014 WHO was notified of an outbreak in Guinea of a disease causing fever, severe diarrhea and vomiting, with a high fatality rate. Laboratories identified Zaire ebolavirus. Sequencing of full genomes showed that the Guinean virus formed its own clade, separate from the strains known from the Democratic Republic of Congo and Gabon. The outbreak had begun in the forested rural southeast of the country, and Sylvain Baize and colleagues linked the confirmed cases to a presumed first death in December 2013.

The virus reached Liberia and Sierra Leone, where weak surveillance and fragile health systems let it spread. On 8 August 2014 the WHO Director-General declared a Public Health Emergency of International Concern on the advice of an Emergency Committee. The committee cited the virus's virulence, intense transmission in communities and health facilities, severe shortages in the health systems of the affected countries, several generations of spread in capital cities, and many infections among health workers, a sign that infection control was failing.

CDC counts 28,610 cases and 11,308 deaths in the three countries, about 40 percent of recorded cases, making it the largest Ebola outbreak in history. By 27 March 2016 Guinea had joined Liberia and Sierra Leone in meeting the criteria for confirming that their original chains of transmission were broken. Two days later WHO ended the emergency and its temporary recommendations. Twelve new clusters had appeared by then, each detected and responded to rapidly, and the outbreak was declared over in June 2016.