Editor’s note: This article was originally published on May 19 and has been updated to incorporate new information.

The Centers for Disease Control and Prevention (CDC) is investigating five monkeypox cases in the United States, while the World Health Organization (WHO) has now confirmed 92 cases in around a dozen countries.

So far, these cases have “mainly but not exclusively been identified amongst men who have sex with men,” according to the WHO. Nonetheless, others should be aware that they, too, could be at risk. Experts stress that monkeypox does not spread as easily as SARS-CoV-2, the virus that causes COVID-19, and they do not expect a pandemic of that scale. The new cases have so far been mild, with no deaths reported.

The U.K. Health Security Agency (UKHSA) reported the first case in the current outbreak on May 7, a man who had recently traveled to Nigeria, where monkeypox is endemic. This was soon followed by two additional cases who share a household and four cases among gay and bi men, all of whom appear to have contracted the virus locally. As of May 23, UKHSA had reported 56 confirmed cases. A large number of confirmed cases have also been reported in Spain, Portugal and Canada, with smaller numbers in several other European countries, Australia and Israel.

Worldwide, all but one of the more than 250 confirmed or suspected cases compiled by Global.health with a known age and sex are young or middle-aged men; one woman in Spain is a suspected case. Many of the men identify as gay or bisexual or were seen at sexual health clinics. Several cases are reported to be linked to a sauna in Spain and events including a fetish festival in Belgium.

In the U.S., the CDC has confirmed one case in Boston, with four more presumptive cases in New York City, south Florida and Salt Lake City, Capt. Jennifer McQuiston, DVM, deputy director of the CDC’s Division of High-Consequence Pathogens and Pathology, said during a May 23 media briefing. All are men with a recent history of international travel. (Local testing can show that a person has a virus in the orthopox family, but further testing at the CDC is needed to confirm a monkeypox diagnosis.)

Symptoms and Transmission 

Monkeypox, which is related to smallpox but less severe, typically causes flu-like symptoms (fever, fatigue, muscle aches), swollen lymph nodes and a rash that can occur on the face, genitals, palms of the hands, soles of the feet and elsewhere on the body. The rash, which can be flat, raised, or pus-filled, may resemble other conditions including herpes, syphilis or chickenpox. The virus has an incubation period of up to three weeks before symptoms start, and the illness typically lasts two to four weeks.

The monkeypox virus is transmitted through close personal contact, including skin-to-skin contact, kissing, contact with clothes or bedding and respiratory droplets, but it does not appear to spread through the air like SARS-CoV-2. It is not known whether it is directly transmitted through semen. The virus can enter the body through the respiratory tract, broken skin or mucous membranes, according to the CDC. Experts have historically thought that monkeypox is not easily transmitted between humans, and it is not yet clear why it is now spreading more extensively.

“Monkeypox is not a sexually transmitted infection in the typical sense, but it can be transmitted during sexual and intimate contact,” CDC epidemiologist John Brooks, MD, said during the media briefing. McQuiston added that close personal contact can include household members and health care workers, “but not passing a person in a grocery store.” She noted that transmission is most likely when a person is symptomatic; respiratory transmission is thought to occur when an individual has monkeypox lesions in their mouth or throat.

“Many of these global reports of monkeypox cases are occurring within sexual networks. However, healthcare providers should be alert to any rash that has features typical of monkeypox," Inger Damon, MD, PhD, director of the Division of High-Consequence Pathogens and Pathology, said in a May 18 statement.

Health officials have alerted gay and bi men, in particular, to be on the lookout for unusual rashes. But some fear emphasizing the gay connection could lead to the kind of blame and stigma seen during the early years of the HIV epidemic.

“Close contact is not only sexual contact,” said Boghuma Titanji, MD, PhD, an infectious disease specialist at Emory University in Atlanta. “If you are at a crowded concert, bar, or club, body to body with other people, that’s close contact too. All forms of sexual contact are close contact. Infectious pathogens flourish with the right timing and opportunity. That’s how outbreaks occur.”

Prevention and Treatment 

Monkeypox is not a new disease. Despite its name, it is more commonly associated with rodents. Although mostly seen in Central and West Africa, it has previously been detected in the U.S., including two isolated cases in travelers reported last year. An outbreak that involved nearly 50 people in the Midwest in 2003 originated with exotic pets imported from Africa and spread to captive prairie dogs before jumping to humans, but no person-to-person transmission was confirmed.

After the first new cases were identified in the U.K., doctors and health officials knew what to look for, and case reports began to multiply rapidly, similar to what has happened with an unusual—and still unexplained—global pediatric hepatitis outbreak.

And like that outbreak, the monkeypox cases have spurred much speculation in the press and social media, including questions about whether they might somehow be connected to COVID. Some people, for example, wonder whether SARS-CoV-2 infection could lead to impaired immunity that increases susceptibility to other viruses. Others have suggested the monkeypox virus may have evolved to become more easily transmissible between humans, but so far genetic sequencing does not support this.

People with monkeypox usually recover without treatment. The strain involved in the current outbreak has a fatality rate of around 1%, though this is closer to 10% for a different strain in Africa. Monkeypox is more likely to cause severe illness in children and immunocompromised people. However, Brooks said that people living with HIV who have viral suppression and a robust CD4 T-cell count should not be at increased risk.

Antiviral medications used to treat smallpox can also be used for monkeypox, including tecovirimat (TPOXX), which prevents the virus from exiting cells to infect new ones, and the nucleoside analogues cidofovir (Vistide, familiar to people with HIV as a treatment for cytomegalovirus retinitis) and brincidofovir (Tembexa).

Smallpox vaccination can prevent monkeypox as well, and cases have risen during the past few decades since that vaccine was discontinued. In the U.S., routine smallpox vaccination was halted in the early 1970s after the disease was brought under control. This means only people around age 50 or older have vaccine-induced immunity, which may have waned over time. In 1980, the WHO declared that naturally occurring smallpox had been eradicated worldwide.

Monkeypox can be controlled through ring vaccination or targeted vaccination of close contacts of an infected person. Smallpox vaccines are now being given to high-risk contacts of known cases and health care providers. Because the monkeypox incubation period is long, vaccines can be administered before or up to two weeks after exposure.

The new outbreak has spurred calls for resuming universal smallpox vaccination, but experts do not recommend it at this time for the population at large. An older live vaccinia virus vaccine (ACAM2000) can cause adverse events, especially in people with compromised immunity, so it is not suitable for mass vaccination against monkeypox. In September 2019, the Food and Drug Administration approved a new, safer non-replicating smallpox and monkeypox vaccine (Jynneos) that contains modified vaccinia virus Ankara. The U.S. maintains a national stockpile of smallpox vaccines in case of bioterrorism.

While researchers and public health officials race to learn more about the ongoing outbreak, monkeypox transmission can be reduced through familiar precautions such as handwashing, avoiding close personal contact and wearing a high-grade mask if face-to-face with an infected person. But again, the virus is not known to spread over longer distances through the air.

Brooks advised people who have possible monkeypox symptoms, including flu-like illness or an unusual rash, to forgo social contact and seek care.

“I don’t think at the present time there is sufficient evidence of spread occurring so rapidly that we want to shut down any events or recommend that things be postponed,” he said at the briefing. “On the contrary, we want to empower people to take the initiative to hold themselves back from participating if they’re feeling ill and to seek evaluation if after an event they start feeling ill.”