After the Sept. 11 attacks, the United States considered immunizing the entire population to protect against a terrorist attack using smallpox. “In the end, it was decided no, because of the negative consequences of vaccinating lots of people,” Bill Hanage, an epidemiologist at the Harvard T.H. Chan School of Public Health.

“Vaccine side effects are rare,” he added. “But once you start giving it to millions of people, then they will start to add up.”

Newer generation vaccines like Jynneos are likely to be safer for large groups, and ring vaccination may be enough to contain the virus. “Hopefully, presumably, monkeypox is still relatively rare right now, and a ring vaccination strategy may well be able to keep it completely at bay,” Dr. Hanage said.

In addition to vaccines for prevention, the United States has procured more than two million doses of an antiviral pill called tecovirimat, which is approved to treat smallpox in those who become infected, according to the C.D.C. The agency is also working with the drug’s manufacturer to develop an intravenous form.

Human monkeypox was first identified in 1970 in a 9-year-old boy in a region of the Democratic Republic of Congo where smallpox had been eliminated. Monkeypox cases in the country have significantly increased in the decades since smallpox mass vaccination ended.

In 2003, the United States recorded dozens of monkeypox cases that were traced to infected pets. Although the virus was first discovered in 1958 in monkeys kept for research purposes, it is spread by rodents.

A week to two weeks after exposure, infected people may begin to experience fever, sore throat, cough, fatigue and body aches. They also develop a distinct rash, first on the face, then on the palms of the hand and soles of the feet, and then all over the body. The lesions blister, grow and fill with a white puslike substance.