So how worried should the world be about monkeypox?
Disease experts warn that, while it may not be about to trigger another Covid-like pandemic, it is likely to spread further and needs to be taken seriously.
'Serious outbreak must be taken seriously'
Jimmy Whitworth, a professor of international public health at the London School of Hygiene and Tropical Medicine, said that meant the current international outbreak was "highly unusual".
He told Reuters: "Historically, there have been very few cases exported. It has only happened eight times in the past before this year."
While he said the cases are not likely to cause a nationwide epidemic like Covid, he warned that it is "a serious outbreak of a serious disease – and we should take it seriously".
Matthew Ferrari, director of the Centre for Infectious Disease Dynamics at Penn State University, added: "What we're seeing may not manifest as a large-scale catastrophe. But we're seeing a predictable phenomenon that is a stark reminder that emergence events are not black swans and we must plan accordingly."
By Thursday (May 20), the UK had logged 20 cases, Portugal had recorded 23, Spain 30. The United States and Sweden have each reported one. Italian authorities have confirmed one case and suspect two more.
The World Health Organisation (WHO) is warning that festivals and parties in the summer "could accelerate" transmission as cases are being detected in people engaging in sexual activity.
A Spanish sauna, a rendezvous for gay men to find and engage in sex, has also been cited as a likely superspreader event for the virus in the European country. The only approved vaccine for smallpox, which also works against monkeypox, is made by a German company Bavarian Nordic, which this week announced an "undisclosed European country" had ordered fresh doses of the vaccine in response to the outbreak.
Health Secretary Sajid Javid also confirmed this week the UK had ordered more doses of a smallpox vaccine, believed to be the Imvanex jab made by Bavarian Nordic.
Disease experts say the wide array of new cases is one source of concern and points to the virus having been already spreading undetected for some time.
Only the first of the UK victims had recently travelled to Africa, meaning the others caught it from domestic spread. Two of the cases live together, but there's no direct contact between the rest.
Bill Hanage, an epidemiology professor at Harvard, said: "These are the largest, most disseminated outbreaks of their kind we have seen. It is obvious that we should be making protective vaccines now."
He wrote on Twitter that it was "very plausible that transmission has been happening for some time unnoticed because folks don't expect to see monkeypox and so don't diagnose it".
He went on: "Now on the bright side, this implies that the severity of the infections are not extremely high or we would have noticed sooner. On the gloomier, it means there are multiple transmission chains out there already.
"That means lots of infections and, as we should have learned from recent experience, even if an infection is mostly mild that is scant comfort if you get so many of them the rare poor outcomes add up."
Monkeypox is a rare, usually mild infection, related to the dreaded smallpox which was eradicated in 1980. Monkeypox is typically caught from infected wild animals in parts of Africa.
Symptoms usually start after five to 21 days and include a fever, a headache, muscle aches, backache, swollen glands, shivering and exhaustion.
A chickenpox-like rash of raised spots often starts on the face and spreads, turning into small blisters filled with fluid. These blisters eventually form scabs which later fall off, according to the NHS website.
In more extreme cases, the disease can sometimes be fatal. There are two varieties: a Congo strain, which is more severe and may kill up to one in 10, and a West African strain, which has a fatality rate in about 1 per cent of cases. The UK cases have all been the West African strain.
Fatality statistics have often been compiled from outbreaks where healthcare is often patchy, however, and the prognosis for patients with American or European treatment is expected to be much better.
Monkeypox gained its name because it was first discovered in 1958 in monkeys kept for research, but signs of infection have also been found in various rats, mice and squirrels. Such wild rodents are thought to act as the main wild host for the virus and infection then occasionally spills over into humans. The first human case was recorded in 1970.
The disease spreads from animals to humans by bites, or by touching an infected animal's blood, body fluids or fur, or eating its partially cooked meat. Monkeypox does not spread easily between people, but the WHO says it is possible to catch via "close contact with respiratory secretions, skin lesions of an infected person or recently contaminated objects".
In practice, that can mean through touching clothing, bedding or towels used by someone with the rash, as well as touching monkeypox blisters or scabs, or getting too close to coughs and sneezes from an infected person.
Of the 20 UK cases recorded so far, most are in men who are gay, bisexual or who have sex with men. That has led to speculation that the disease may now be sexually transmitted, but experts have said it is probably just spreading through close contact.
Dr Michael Head, a senior research fellow in global health at the University of Southampton, said: "There is no evidence that it is a sexually transmitted virus, such as HIV. It's more that here the close contact during sexual or intimate activity, including prolonged skin-to-skin contact, maybe the key factor during transmission."
Hope on the horizon
The good news is there is already an effective vaccine against monkeypox. Jabs which were widely delivered to eradicate smallpox, caused by the variola virus, are also up to 85 per cent effective against monkeypox, according to the WHO.
The US alone has enough smallpox vaccine stockpiled to give to everyone in the country. The UK Government has also started giving jabs to health workers and others who may have been exposed.
Widespread use of the vaccine has been wound down, however, since smallpox was stamped out more than four decades ago – and this may be a factor in the current spread of monkeypox.
"Two factors probably contribute to the cases we have seen over recent years," said Dr Michael Skinner, a reader in virology at Imperial College London.
"One is that the cessation of smallpox vaccination after the global eradication of variola virus in 1980 means that levels of cross-reactive immunity to monkeypox have now waned or disappeared, so that people can now be infected with monkeypox virus."
The other possibility is that something has changed to widen or broaden the distribution of the virus among West Africa's wildlife, meaning humans are more likely to catch it.
"We're seeing a shift in the age distribution of cases," Michael Ryan, the WHO's executive director for health emergencies, told US health news website Stat.
"We're seeing a shift in the geographic distribution of cases. We have to really understand that deep ecology. We have to really understand human behaviour in those regions and we have to try to prevent the disease from reaching humans in the first place."
Public-health experts are now scrambling to trace cases and trying to ensure that doctors and nurses are aware of the disease and can report infections if necessary.
"This cat is already out of the bag," said Harvard's Prof Hanage. "All that remains is to see what sort of cat it is – a tabby or a tiger?"
