Dozens of reported cases of monkeypox have been reported outside of areas in sub-Saharan Africa (namely, Nigeria, Central African Republic, Cameroon, and Democratic Republic of Congo) in which it is endemic. As of 12PM EST on 19 May, there are 75 confirmed or suspected monkeypox cases across the UK, Portugal, Spain, Canada, Sweden, Italy, and the US according to a spreadsheet maintained by Global.health. According to a 19 May report by the the European Centre for Disease Prevention and Control, many of these cases are likely the result of community transmission since many do not appear to be linked to travel:

Given the unusually high frequency of human-to-human transmission observed in this event, and the probable community transmission without history of traveling to endemic areas, the likelihood of further spread of the virus through close contact, for example during sexual activities, is considered to be high. The likelihood of transmission between individuals without close contact is considered to be low.

Monkeypox is closely related to but distinct from smallpox. Smallpox was eradicated in 1980. The first case of monkeypox was diagnosed in 1970 and in recent years the frequency and geographic distribution has increased in Central Africa and West Africa. A recent systematic review finds that monkeypox has a less severe clinical presentation and is less transmissible than smallpox, but also that it has been evolving in recent decades to become of "global relevance." The same article also reports a pooled case fatality rate estimate of 8.7%, with a point estimate of 10.6% for the Central African clade and 3.6% for the West African clade. As of 18 May, all sequenced monkeypox cases seem to be of the West African clade according to the World Health Organization (WHO).

Prior to the ongoing 2022 outbreak, the majority of reported and suspected monkeypox cases were thought to be the result of animal-to-human transmission rather than human-to-human transmission. The secondary attack rate (SAR) estimates of monkeypox pre-2022 are consistent with this — according to the aforementioned systematic review article, which reviewed 16 articles with SAR estimates:

More than half of the articles (9/16) reported an SAR of 0%, and this spanned the decades from the 1970s through 2010–2019. Similarly, over those same five decades, the SAR ranged from 0.3–10.2% in 6/16 articles . In the remaining article, a median SAR of 50% was reported in an outbreak among 16 households.

However, monkeypox now seems to be spreading as a result of close contact between humans, perhaps in particular during sexual activities.

Under the 2005 International Health Regulations, WHO has the power to decide when to declare a Public Health Emergency of International Concern (PHEIC). A PHEIC is an “extraordinary event that constitutes a public health risk to other countries through international spread of disease and potentially requires a coordinated international response.”.

Since 2009, WHO has made six PHEIC declarations, the most recent of which was the 30 January 2020 PHEIC declaration of COVID-19.