As monkeypox cases continue to grow worldwide, the World Health Organization (WHO) last week called for a "unified response" against the current outbreak.

How big is the threat from monkeypox? Here's what experts say.

Where monkeypox stands right now

Currently, more than 2,100 monkeypox cases have been reported worldwide in 42 different countries, including one confirmed death in Nigeria. Europe has been most significantly affected with the European Centre for Disease Prevention and Control identifying 1,704 cases in the region through June 15.

In the United States, there are 142 confirmed and suspected monkeypox cases across 23 states and the District of Columbia. Over the weekend, Indiana, Missouri, and the city of Houston all reported their first probable cases.

According to Jimmy Whitworth, from the London School of Hygiene and Tropical Medicine, the rising number of cases worldwide suggest that there may "have been some superspreading events that have enabled infections to spread rapidly into many different countries, and there are chains of transmission that suggest that there might have been some cases without any obvious signs of infection, which has allowed further spread which has not been detected."

Going forward, WHO said it was removing the distinction between non-endemic and endemic countries "to reflect the unified response that is needed" to combat the current monkeypox outbreak. This week, WHO will convene to decide whether the outbreak should be declared an international public health emergency.

Why monkeypox is primarily affecting MSM

In a report, WHO noted that the current monkeypox outbreak has primarily affected "men who have sex with men [MSM] who have reported recent sex with new or multiple partners" and that most cases "have presented through sexual health or other health services in primary or secondary health care facilities, with a history of travel primarily to countries in Europe, and North America or other countries."

Although monkeypox can be transmitted by anyone, the highly interconnected sexual networks within the MSM community may allow the virus to spread more easily than it would in the general population, Science reports.

"It's entirely possible for this epidemic to rage among a subset of people just because that subset is connected in a network differently than everyone else," said Keletso Makofane, a social network epidemiologist at the FXB Center for Health and Human Rights at Harvard University.

Although many health officials are concerned about potentially stigmatizing MSM, other experts say that it is necessary to convey the risks of monkeypox to the MSM community to reduce the spread of the virus and increase vaccination among those who would benefit the most.

"We should say: It's not about who you are. It's about what you're doing," said Gregg Gonsalves, an epidemiologist at the Yale School of Public Health and a former HIV activist. "And we're not going to stigmatize it. But just know that you're at greater risk if you fit this profile."

Could monkeypox jump to an animal reservoir?

So far, it's not clear if or when the current monkeypox outbreak will end, but one potential concern is that the virus will make the jump from humans to wildlife and become endemic in an animal reservoir, which could then lead to periodic outbreaks.

If monkeypox finds an animal reservoir in the United States, much like the coronavirus has with white-tailed deer, the virus will be "with us forever," said Barbara Han, a disease ecologist at the Cary Institute.

According to The Atlantic, monkeypox has been able to infect several animal species, including sun squirrels, Gambian pouched rats, dormice, anteaters, rabbits, prairie dogs, and some primates. However, active virus has only been found in wild animals twice so far—once in a rope squirrel in the Democratic Republic of Congo in the 1980s, and once in a sooty mangabey in Côte d’Ivoire around a decade ago.

Jeffrey Doty, a disease ecologist at CDC, said rodents are likely "responsible for maintaining this virus in the wild," but so far, a specific animal reservoir for monkeypox has not been confirmed.

Currently, most disease experts consider the possibility of monkeypox becoming endemic in wild animals in the United States unlikely, though there is still some risk that it could happen. As a precaution, CDC has recommended infected patients avoid interacting with animals, including pets and livestock.

Right now, the best way to prevent the spread of monkeypox from humans to animals is "to control the human outbreak," Han said, which will ultimately reduce the virus's ability to spread to new hosts and expand the number of species it can infect. (Wappes, CIDRAP News, 6/20; Carbajal, Becker's Hospital Review, 6/21; Rose/Ellis, CNN, 6/19; Webb, Houston Chronicle, 6/18; Kupferschmidt, Science, 6/20; Wu, The Atlantic, 6/21)