The World Health Organization is informed of cases of pneumonia with an unknown cause, detected in Wuhan. This later turned out to be COVID-19, the cause of the COVID-19 pandemic.

The World Health Organization (WHO) is a specialized agency of the United Nations which coordinates responses to international public health issues and emergencies. It is headquartered in Geneva, Switzerland, and has 6 regional offices and 150 field offices worldwide. Only sovereign states are eligible to join, and it is the largest intergovernmental health organization at the international level.

Wuhan is the capital of Hubei, China. With a population of over eleven million, it is the most populous city in Hubei and the eighth-most-populous city in China. It is also one of China's nine national central cities.

December 31 is the 365th day of the year in the Gregorian calendar. It is known by a collection of names including: Saint Sylvester's Day, New Year's Eve or Old Year's Day/Night, as the following day is New Year's Day. It is the last day of the year; the following day is January 1, the first day of the following year.

Coronavirus disease 2019 (COVID-19) is a contagious disease caused by the coronavirus SARS-CoV-2. In January 2020, the disease spread worldwide, resulting in the COVID-19 pandemic.

The COVID-19 pandemic, caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), began with an outbreak of COVID-19 in Wuhan, China, in December 2019. Soon after, it spread to other areas of Asia, and then worldwide in early 2020. The World Health Organization (WHO) declared the outbreak a public health emergency of international concern (PHEIC) on 30 January 2020, and assessed the outbreak as having become a pandemic on 11 March.