A rare disease, monkeypox is now spreading in the United States for the first time, with cases climbing in most parts of the country. Public health officials are using a combination of public education and vaccines to try and stem the spread of the illness.

The virus that causes monkeypox is related to smallpox, and while the smallpox vaccine may provide some protection, it hasn’t been widely administered in the U.S. for decades. Monkeypox was discovered in 1958 when two outbreaks of a pox-like disease occurred in colonies of monkeys kept for research. Despite its name, the source of the disease remains unknown. The virus can be carried by certain rodents and primates, including people; until now, infections were limited to residents of parts of Africa and people who had traveled there.

By what epidemiologists say was a quirk of fate, gay men are currently most at risk of contracting the disease. The current outbreak appears to be related to two raves held recently in Spain and Belgium and attended primarily by gay men, according to a researcher who advises the World Health Organization. While the virus was likely transmitted via sexual contact, it can be transmitted by other kinds of close physical contact and does not distinguish between men and women or their sexual orientation.

The disease is expressed in flu-like symptoms that include soreness, a fever and swollen lymph nodes, eventually accompanied by a pimple-like rash.

“It’s difficult during the first five days” to know whether it’s the flu, COVID, monkeypox or some other illness,” said Dr. Andrew Jorgensen, chief medical officer with Outer Cape Health Services. During that time period, “it’s sort of good advice anyway: if you’re sick, stay home,” he said. If no rash appears after five days, the illness is probably not monkeypox. As always, people should check in with their healthcare provider if they’re uncertain.

Monkeypox is spread by close, personal, usually skin-to-skin contact with an infected person’s rash, scabs or body fluids. Transmission is also possible by touching objects like clothing or bedding that have been used by an infected person. It can happen during sex, skin-to-skin hugging, massage, kissing or prolonged face-to-face contact, according to the CDC. Transmission can happen from the onset of symptoms until the rash has fully healed and a fresh layer of skin has formed, usually two to four weeks.

While monkeypox infections are rarely fatal, “it can be very, very painful,” Jorgensen said. “It’s not just like having poison ivy.” The rash is more akin to shingles, and people with confirmed cases are required to quarantine for three weeks. Staying away from work for that long, “especially if you’re living on Cape Cod in the summertime, can be financially devastating to you,” he said. For those reasons alone, public health officials are eager to stem the spread of monkeypox.

As of Monday, Massachusetts had 134 confirmed cases of the disease. Limited doses of a vaccine are available, and are being administered at clinics in Provincetown, Boston, and around a half-dozen other communities in the state. Those eligible for the vaccine include people known by public health officials to have been exposed; their sexual partners, household contacts and healthcare workers; and those who have had multiple sexual partners in the previous two weeks in a community where monkeypox is known to exist, including Provincetown.

“It’s sort of a limited population right now,” Jorgensen said.

In a “pretty herculean effort” for a small community health center, Outer Cape Health Services has vaccinated nearly 2,000 individuals so far this summer. “I’m very proud of that effort,” Jorgensen said. Vaccine eligibility is likely to expand when more doses become available, just as happened initially with the COVID vaccine. Initially, those shots were restricted to people with high-risk health conditions and the elderly. “We’re doing the same thing with the monkeypox vaccine,” he said.