The lone U.S. case was confirmed in an adult male who had recently traveled to Canada, the Massachusetts Department of Public Health said in a news release. The department is completing contact tracing, and the man is hospitalized in good condition, the agency said.

Also late Wednesday, the CDC informed Fortune that the United Kingdom had notified the U.S. of eight individuals who may have been seated near an infected U.K. traveler when they flew from Nigeria to London from May 3-4. Of the eight, one is no longer in the U.S. and another was not a contact, the health agency said.

“The remaining six are being monitored by their respective state health departments,” the CDC said in a statement. “None of these six travel contacts have monkeypox symptoms, and their risk of infection is very low.”

CDC epidemiologist Andrea McCollum told Fortune late Wednesday that she isn’t particularly concerned about outbreaks, but that potential additional U.S. cases are likely.

“It’s not highly transmissible, and it’s not likely to cause large outbreaks,” she said. “We know how to contain it and vaccinate against it. The only reason to be worried is if you think you had close contact with somebody who has a known case. If those people develop fever or a rash, they should go get it checked out.”

A milder version of smallpox

Monkeypox, a rare disease related to smallpox and cowpox, was first identified in 1958 among colonies of monkeys kept for research, according to the CDC. African rodents and non-human primates like monkeys may harbor the disease and infect people. The first human case was recorded in 1970, in the Democratic Republic of Congo, where the majority of infections now occur. It has since been reported in humans in other African countries like Cameroon, Central African Republic, and Ivory Coast.

Prior to this year’s outbreak in Europe, cases had been reported in the U.S., in addition to Israel, Singapore, and the United Kingdom, according to the CDC. Last year two cases were identified in the U.S., one in November and one in July. Both involved travelers who had just returned from Nigeria. Human monkeypox was first reported outside of Africa in the U.S. in 2003, when 47 confirmed and probable cases were reported. All cases had contact with pet prairie dogs that had been housed near small mammals—including rope squirrels, tree squirrels, and African giant-pouched rats—imported from Ghana.

Symptoms are similar to, but milder than, that of smallpox, according to the CDC. Initial symptoms usually include fever, headache, muscle aches, and exhaustion. Within one to three days, patients develop a rash, usually on the face and then spreading to other parts of the body. Lesions progress through various stages before scabbing. The illness usually lasts two to four weeks. The typical incubation period is 7 to 14 days but can range from 5 to 21 days.

Monkeypox has caused death in as many as 10% of patients in Africa, the CDC states.

‘Multiple chains’ of transmission in the U.K.?

On Wednesday Portuguese health authorities confirmed five cases in young men, and Britain announced another two, one in London and another in southeast England, the Associated Press reported. Portugal’s General Directorate for Health said it was investigating an additional 15 suspected cases, all identified this month in or around Lisbon, the capital.

All Portuguese patients are men, most of them young, and have skin lesions. All are in stable condition, the AP reported Wednesday, adding that Portuguese authorities did not offer details on their travel history, if any.

Neither of the British patients announced Wednesday had traveled to Africa. They also have no known links to other previously confirmed patients, suggesting that “multiple chains” of monkeypox transmission may be occurring in the country, the AP reported.

The cases confirm “our initial concerns that there could be spread of monkeypox within our communities,” Dr. Susan Hopkins, chief medical adviser of Britain’s Health Security Agency, told the AP, adding that recent cases have been seen “predominantly in gay, bisexual, or men who have sex with men,” though the mode of transmission is currently unclear.

Is sex a new mode of transmission?

Earlier in the week the health agency reported four additional cases, among gay and bisexual men in London, according to the AP. The virus has not previously been documented as spreading through sex.

Transmission of monkeypox from human to human has been thought to occur mainly through large respiratory droplets, meaning that “prolonged face-to-face contact is required,” according to the CDC. Other modes of transmission have been thought to include contact with bodily fluids or lesions, or contact with contaminated clothing or linens.

“The recent cases suggest a potentially novel means of spread,” Neil Mabbott, a disease expert at the University of Edinburgh, told the AP, adding that similar viruses are known to spread via sexual contact.

Spanish health authorities said late Wednesday they were assessing another 23 potential cases, noting all were young men and that most had sex with other men, the AP said, adding that Britain had previously reported three cases.

Jynneos, an attenuated live-virus vaccine, has been approved by the U.S. Food and Drug Administration for prevention of monkeypox and smallpox. In the event of an outbreak in the U.S., the health agency would establish guidelines as to who should be vaccinated, the CDC says on its website.

Several antivirals are available for treating monkeypox, but only limited data is available regarding their use in humans with the condition, according to the CDC.

Editor’s note: This article was updated at 7:42 p.m. ET May 18 to reflect that the six individuals in the U.S. who were potentially exposed to monkeypox May 3-4 are being monitored by public health officials.