The rare monkeypox virus, usually confined mostly to Central and West Africa, has spread in unusual ways this year, and among populations that have not been vulnerable in the past.

But while the transmissions have created some alarm among officials and infectious disease experts, and while a Covid-weary world is on high alert for new outbreaks, there are several reasons monkeypox is not being treated with the same level of concern as the coronavirus.

Here’s what to know about monkeypox and the risks it poses.

What is monkeypox?

Monkeypox is a virus endemic in parts of Central and West Africa. It is a more benign version of smallpox.

It was discovered in 1958, after outbreaks occurred in monkeys kept for research, according to the Centers for Disease Control and Prevention.

What are the symptoms?

Monkeypox creates a rash that starts with flat red marks that become raised and filled with pus. Infected people will also have a fever and body aches.

Symptoms typically appear in six to 13 days but can take as long as three weeks after exposure. They can last for two to four weeks, with severe cases occurring more commonly among children, according to the World Health Organization.

The C.D.C. says there is “no proven, safe treatment” for monkeypox but notes that, for the purposes of controlling an outbreak in the United States, smallpox vaccines and other treatments can be used.

How infectious is it?

Typically it does not lead to major outbreaks — in most years there are just a handful of cases outside Africa, if any. The most severe outbreak in the United States came in 2003, when dozens of cases were linked to exposure to infected prairie dogs and other pets. It was the first time there had been a monkeypox outbreak outside of Africa, according to the World Health Organization.

Within Africa, 11 countries have reported cases since 1970, when the first human case was identified in a 9-year-old boy in the Democratic Republic of Congo. Nigeria has experienced a large outbreak, with more than 500 suspected cases and 200 confirmed cases since 2017, the W.H.O. said.

The virus can spread via body fluids, skin contact and respiratory droplets. The majority of cases this year have been in young men, many of whom self-identified as men who have sex with men.

“Most cases presented with lesions on the genitalia or peri-genital area, indicating that transmission likely occurs during close physical contact during sexual activities,” the European Center for Disease Prevention and Control said on Friday.

How many cases have there been this year, and where have they been?

There have been 38 cases worldwide this year as of Thursday, including 37 with no history of travel to endemic countries, according to the European Centre for Disease Prevention and Control. Britain reported an additional 11 cases on Friday.

In the United States, the first case of 2022 was diagnosed in Massachusetts on Wednesday. The man had recently traveled to Canada, which has had two cases this year. New York City health authorities announced on Thursday that they were investigating a possible case.

Europe has been hit much harder. As of Thursday, Portugal had reported 17 cases, Spain has had seven, Belgium has had two, and France, Italy and Sweden have each had one.

Britain had reported nine cases as of Thursday, but Sajid Javid, Britain’s health secretary, said on Friday that the number had risen to 20. The W.H.O. said on Thursday that the country’s infections appeared to be locally acquired, but “the extent of local transmission is unclear at this stage and there is the possibility of identification of further cases.”

None of the infected people have died, according to the European Centre for Disease Prevention and Control.

How is the current outbreak different?

This is the first time that chains of transmission were reported in Europe without links to West or Central Africa, according to the European Center for Disease Prevention and Control. The agency also said this year’s cases included the first that have been reported among men who have sex with men.

Should I be worried?

The likelihood of the virus being spread during sexual contact is high, but the risk of transmission from other forms of close contact is low, the European Center for Disease Prevention and Control said.

The symptoms are typically mild, and most people recover within weeks, but the virus has had a fatality rate of about 3.3 percent in Nigeria, with children, young adults and immunocompromised people most susceptible.