Monkeypox is not highly contagious, scientists say. So why is it suddenly in nine countries?

This 2003 electron microscope image made available by the Centers for Disease Control and Prevention shows a monkeypox virion, obtained from a sample associated with the 2003 prairie dog outbreak.Cynthia S. Goldsmith, Russell Regner/Associated Press

Even before health officials reported Wednesday that a Massachusetts man was hospitalized with monkeypox virus, scientists were puzzled by outbreaks of the rare disease in Europe. The virus, first discovered in monkeys in the 1950s, is rarely seen outside Central and Western Africa. When it is, it’s usually linked to African travel. But now that’s changed. Here’s the latest.

How many cases have been reported?

More than 100 confirmed and suspected cases across nine countries have been reported as of late Thursday by HealthMap, a team of researchers, epidemiologists, and software developers at Boston Children’s Hospital that provides real-time surveillance of emerging public health threats. The first case was confirmed in England on May 6. There are now nine cases in the United Kingdom, at least 29 in Portugal, and 17 in Canada, as well as cases in Spain, France, Sweden, Italy, and Belgium. The Boston case is the first to be identified in the United States. On Thursday, the New York City Department of Health and Mental Hygiene said it was investigating a second suspected case. Also on Thursday, Montreal officials said they had found links between some of their suspected cases and the Massachusetts case, who reported traveling to Canada before falling ill.

So what is monkeypox?

The first human case of monkeypox was recorded in 1970 in the Democratic Republic of the Congo, according to the US Centers for Disease Control and Prevention. African rodents and nonhuman primates (like monkeys) may harbor the virus and infect people. The infection, which typically starts as a flu-like illness with fever and swelling of the lymph nodes, progresses to a rash that develops into distinct fluid-filled lesions. Unlike other similar illnesses, monkeypox classically involves a rash on the palms of your hand and soles of your feet.

The time from infection to first symptoms is usually one to two weeks but can take up to 21 days. The illness typically lasts for 2−4 weeks. In Africa, monkeypox has been shown to cause death in as many as 1 in 10 persons who contract the disease.

There are two types, or clades, of monkeypox: the West African clade, which has a case-fatality rate of 1 percent. A second clade, called the Congo Basin, has a case-fatality rate of 10 percent. The Massachusetts case is West African, officials said, as are the cases in the UK.

How does monkeypox spread?

The virus typically spreads through direct contact with an infected animal through a bite or scratch or between people through bodily fluids and contact with fluids from rash lesions. But it’s most often thought to spread through respiratory droplets that don’t travel more than a few feet and don’t hang in the air, unlike droplets from the virus that causes COVID-19.

“It’s really someone coughing and in close proximity. It’s not something that you are going to get just walking by someone,” said Dr. Daniel Bausch, senior director for emerging threats & global health security at FIND, a Geneva health foundation. “This is not a source of panic. But we are still trying to put this all together and it’s probably going to be the biggest epidemic of monkeypox we have had in Europe and North America.”

Are the cases related?

Some appear to be, but scientists have so far been unable to establish common links across so many countries and are still trying to understand how these cases may or may not be related. “The geographic dispersion of this is quite rare,” Bausch said.

Another puzzling aspect of this outbreak is how fast the cases and symptoms have been popping up, leading some to wonder if this is a new strain of monkeypox with a shorter incubation period.

“Either something is fundamentally different here, or there are more to these cases than we understand,” said Dr. Meghan May, professor of infectious disease at the University of New England College of Medicine in Maine.

Is monkeypox a sexually transmitted disease?

The virus has not typically been known to spread through sexual contact. But many of the confirmed and suspected cases across several of the countries are among men aged 20 to 55 who report having sex with other men. And many of the reports have come from health facilities that provide care for patients with sexually transmitted diseases.

Fenway Health, a Boston company long known for providing care for the lesbian, gay, bisexual, and transgender community, has been busy trying to get accurate information about monkeypox to caregivers and patients.

“We are strongly encouraging all people with a genital lesion to seek help with a health care provider,” said Dr. Ami Multani, medical director of infectious disease at Fenway Health. While some sexually transmitted diseases, such as syphilis, HPV, and herpes simplex also produce genital lesions, the rash from monkeypox is “very unique,” she said.

“If someone has chickenpox, you might have a scab here, or [fluid-filled lesion] there,” Multani said. “But in monkeypox it’s all flat, then all become raised, then fluid-filled, then scab over,” roughly all at the same time.

Is there a treatment for monkeypox?

Currently, there is no proven, safe treatment for monkeypox virus infection, according to the CDC. For purposes of controlling a monkeypox outbreak in the United States, health officials have used smallpox vaccines and antivirals. Monkeypox is related to the virus that causes smallpox, a once-dreaded disease that was declared eradicated in 1980.

Before smallpox was eradicated, people routinely were vaccinated against the disease. But such vaccination ended decades ago, and scientists say it’s not known whether those years-old vaccinations would be protective today against monkeypox.

“Smallpox vaccine has not been extensively tested against monkeypox,” said Bausch, the Geneva infectious disease expert. “Studies from nonhuman primates tell us the viruses are similar, but they don’t tell you how long after smallpox vaccine you are, or are not, protected against monkeypox.”