In May 2022, the World Health Organization (WHO) made an emergency announcement of the existence of a multi-country outbreak of mpox, a viral disease then commonly known as "monkeypox". The initial cluster of cases was found in the United Kingdom, where the first case was detected in London on 6 May 2022 in a patient with a recent travel history from Nigeria where the disease has been endemic. On 16 May, the UK Health Security Agency (UKHSA) confirmed four new cases with no link to travel to a country where mpox is endemic. Subsequently, cases have been reported from many countries and regions. The outbreak marked the first time mpox had spread widely outside Central and West Africa. The disease had been circulating and evolving in human hosts over several years before the outbreak and was caused by the clade IIb variant of the virus.