Editor's note: This story was updated on May 23 to reflect the latest case counts.
There's a monkeypox outbreak in the United Kingdom, Portugal, Spain and other European countries. The outbreak is small — so far about 80 suspected cases in England, Spain and Canada as well as Portugal, Sweden, Italy and France. Two suspected cases have also been reported in the U.S.
But health officials have little clue where people caught the monkeypox virus. And there's concern the virus may be spreading through the community — undetected — and possibly through a new route of transmission.
"This [outbreak] is rare and unusual," epidemiologist Susan Hopkins, who's the chief medical adviser of the U.K. Health Security Agency (UKHSA), said in a statement on Monday.
"Exactly where and how they [the people] acquired their infections remains under urgent investigation," the agency said in the statement.
Monkeypox can be a nasty illness; it causes fever, body aches, enlarged lymph nodes and eventually "pox," or painful, fluid-filled blisters on the face, hands and feet. One version of monkeypox is quite deadly and kills up to 10% of people infected. The version currently in England is milder. Its fatality rate is less than 1%. A case generally resolves in two to four weeks.
Typically, people catch monkeypox from animals in West Africa or central Africa and import the virus to other countries. Person-to-person transmission isn't common, as it requires close contact with bodily fluids, such as saliva from coughing or pus from the lesions. So the risk to the general population is low, the U.K. health agency notes.
But in England, the vast majority of the 50-plus cases don't involve recent travel to Africa, suggesting the patients involved in those cases caught the virus in England.
"Presumably this is cryptic spread from an imported case(s)," virologist Angie Rasmussen of the Vaccine and Infectious Disease Organization tweeted on Monday.
In the U.S., one patient, in Massachusetts, had not recently traveled to countries where the disease occurs but had visited Canada.
In addition, there's evidence the virus could be spreading through a new route: sexual contact. "What is even more bizarre is finding cases that appear to have acquired the infection via sexual contact," epidemiologist Mateo Prochazka at the UKHSA tweeted. "This is a novel route of transmission that will have implications for outbreak response and control."
"We are particularly urging men who are gay and bisexual to be aware of any unusual rashes or lesions and to contact a sexual health service without delay," epidemiologist Hopkins said in the UKHSA's statement.
Scientists at the U.S. Centers for Disease Control and Prevention are watching the outbreak in Europe closely. "We do have a level of concern that this is very different than what we typically think of from monkeypox," Jennifer McQuiston, a senior CDC official, told health news site STAT on Tuesday.
In 2019, the U.S. Food and Drug Administration approved the first vaccine for monkeypox, which also protects against smallpox. "This vaccine is also part of the Strategic National Stockpile (SNS), the nation's largest supply of potentially life-saving pharmaceuticals and medical supplies for use in a public health emergency that is severe enough to cause local supplies to be depleted," the agency said in a news release.
A primer on monkeypox
So just what is known about monkeypox? And how threatening is it compared with other emerging viruses?
In 2017, Goats and Soda interviewed two monkeypox experts — Anne Rimoin of the University of California, Los Angeles, and Jay Hooper of the U.S. Army Medical Research Institute of Infectious Diseases — to find out.
Here are some of the questions we asked and some of their surprising answers, updated in light of the current cases.
Where does it come from? Monkeys?
"The name is actually a little bit of a misnomer," Rimoin says. Perhaps it should be called "rodentpox" instead.
The name "monkeypox" comes from the first documented cases of the illness, in 1958, when two outbreaks occurred in colonies of monkeys kept for research, the CDC says on its website.
But monkeys aren't major carriers. Instead, the virus likely persists in squirrels, pouched rats, dormice or another rodent.
How do you catch it?
Primarily, from an animal bite, scratch or contact with the animal's bodily fluid. Then the virus can spread to other people through coughing and sneezing or contact with pus from the lesions.
The lesions from monkeypox are similar to those from a smallpox infection.
"But it doesn't spread very well between people," Hooper says. "Its infection rate is much lower than that of smallpox." In many cases, people don't spread the virus to anyone else.
Up until this current outbreak, a person sick with monkeypox spread the virus to between zero and one person, on average. So all previous outbreaks (up until now) burned themselves out quickly.
"You have primary cases, in which people get monkeypox from an animal, and they may transmit the disease a few generations — but then that's it," she says. "The outbreaks tend to be self-limiting."
"There is no evidence, to date, that person-to-person transmission alone can sustain monkeypox infections in the human population," the World Health Organization's website says.
Scientists don't know yet if the rate of transmission has increased in this current outbreak. If there is enhanced transmission, that could be one reason the current outbreak appears to have spread through the community in three cities.
Has there ever been an outbreak in the U.S.?
"There already was!" Hooper says. "But it was quickly contained."
In 2003, monkeypox hitched a ride with a shipment of animals from Ghana to Illinois. Several giant pouched rats and squirrels tested positive for the virus and eventually spread it to prairie dogs being sold as pets in multiple Midwestern states, the CDC says on its website.
Forty-seven people caught the disease from the prairie dogs. Everyone recovered. And no one spread the disease to another person.
Is monkeypox a "new" virus?
No. The virus has likely been infecting people for centuries, even millennia, Rimoin says. But for a long time, doctors missed the cases.
Monkeypox is closely related to smallpox. "They are clinically indistinguishable," Rimoin says. "So for centuries, doctors have likely mistaken monkeypox for smallpox."
Then in the 1970s, the world was close to eradicating smallpox. Cases plummeted. And doctors in central Africa started noticing another disease that looked like smallpox but didn't spread as well between people. It was monkeypox.
There are several other viruses related to smallpox, including cowpox and camelpox. "I would be more worried about camelpox than monkeypox," Rimoin says, "because that's closer on the genetic tree to smallpox."
Is the disease actually a rising threat? Or are we just better at detecting it?
A little bit of both, Rimoin says.
Back in 2010, Rimoin and her colleagues reported that monkeypox had increased 14-fold in the Democratic Republic of Congo since the 1980s. Incidence rose from less than 1 case per 10,000 people to about 14 cases per 10,000 people.
And the reason for this bump is ironic: the eradication of smallpox.
The smallpox vaccine actually works quite well to protect people against monkeypox. It's about 85% effective (although the smallpox vaccine does have some safety concerns, Hooper points out: "It's a live virus and can cause a deadly infection in people with severely compromised immune systems").
But after the world eradicated smallpox, countries stopped vaccinating kids. And for those who were vaccinated years earlier, their protection has likely waned over time, Hooper says.
"So now there's this growing population of people who don't have immunity to monkeypox," he says. "And when you do have a outbreak, it's likely to be bigger because less people in the community are protected."
That means small monkeypox outbreaks in West Africa and central Africa now involve dozens of cases instead of just one or two, Hooper says.
And in the Democratic Republic of Congo, annual cases have shot up into the thousands. In 2020, there were nearly 4,600 suspected cases, according to a study published in February.
Could the virus become more transmissible and thus more of a global threat?
"Oh, yes," Hooper says. "Every time there's an outbreak — and the more people get infected — the more chances monkeypox has to adapt to people," he says.
In other words, the more time the virus spends inside people, the more time it has to evolve. It could possibly figure out how to spread more quickly among people.
So scientists are keeping a close eye on the virus and outbreaks that occur — especially if the virus appears to change its route of transmission, as may be happening in the current outbreak.
"We didn't think Ebola spread very easily between people," Hooper adds. "And we were all surprised that health care workers could catch it even though they were wearing protective gear."
And of course, many scientists didn't think SARS-CoV-2, the coronavirus that causes the disease COVID-19, would mutate to become more contagious, but that's exactly what has happened in the past two years. SARS-CoV-2 evolved from a virus just about as contagious as the flu virus into one that's almost as contagious as the far more transmissible chickenpox virus.
"With viruses that spill over from animals, you just never know what's going to happen," Hooper says.
And indeed, this new outbreak in Europe may be a sign that the virus has changed — even if just a bit — and may be increasing its ability to spread among people.
Correction: May 18, 2022 12:00 am — An earlier version of this story said a monkeypox case had been reported in the U.S., while later saying there were no cases in the United States. To date, one U.S. case has been reported. We have deleted the sentence saying there were no cases, which was from a version of the story that was published before the U.S. case was reported. Additionally, an earlier version of this story said scientists don't know if the rate of monkeypox transmission has increased, followed by a sentence that said enhanced transmission is one reason that the current outbreak appears to have spread through the community in three cities. To clarify, scientists don't know if the transmission rate has increased; if it has, that could be one reason the outbreak appears to have spread through the community in the three cities.An earlier version of this story mistakenly said Angie Rasmussen's tweet was on Tuesday; it was on Monday, May 16.