The United Nations (UN) Security Council, for the first time in its history, declared a disease outbreak as “a threat to international peace and security” on September 18, 2014. An outbreak of Ebola in West Africa, the most harmful outbreak of this disease ever recorded, gave it cause to do so. The Ebola outbreak also marked the first time that the UN Secretary-General deployed, under his authority, a “United Nations emergency health mission”—neither a peacekeeping operation, nor a “political mission.” Finally, the Ebola outbreak motivated the World Health Organization (WHO) to make use of its authority to declare a “public health emergency of international concern” under the International Health Regulations (IHR) for the third time since their entry into force and induced the WHO Director-General to convene, for the third time in WHO’s history, a special session of the WHO Executive Board to reform WHO’s role in disease outbreak response. The materials presented below form essential background to these important developments in international health law and in the institutional practice of the United Nations system.