The ongoing monkeypox outbreak currently poses a moderate risk to global public health, the World Health Organization said Sunday in a statement that nevertheless raised the specter of the virus becoming entrenched as a pathogen that spreads from person to person.

The WHO said that 23 countries have reported a total of 257 confirmed cases and roughly 120 suspected cases under investigation as of May 26 — a rapid accumulation of cases in an unprecedented outbreak that was first detected earlier this month. To date most of the cases have been diagnosed in Europe and North America. The United States had detected 12 cases as of Friday.

“Currently, the overall public health risk at [a] global level is assessed as moderate considering this is the first time that monkeypox cases and clusters are reported concurrently in widely disparate WHO geographical areas,” the global health agency said.

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“The public health risk could become high if this virus exploits the opportunity to establish itself as a human pathogen and spreads to groups at higher risk of severe disease such as young children and immunosuppressed persons,’’ the statement said, noting that since smallpox vaccination ceased more than 40 years ago, an ever-growing portion of the global population is vulnerable to the monkeypox virus.

Monkeypox is an orthopoxvirus, a family that includes the now eradicated smallpox virus; vaccines and drugs developed to ward off or treat smallpox are expected to offer some protection against monkeypox. Monkeypox triggers milder illness than smallpox did — the latter was fatal in about 30% of cases. The fatality rate for monkeypox is estimated to be between 1% and 10%, with the viruses responsible for the current outbreak, from the West African clade, associated with a fatality rate at the low end of that spectrum.

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The majority of the cases in this outbreak have been detected in men who have sex with men. And while the WHO is asking countries to look for missed cases when they do contact tracing, most of the cases being reported are active, meaning they currently have symptoms, Maria Van Kerkhove, who leads the emerging diseases and zoonoses unit in the WHO’s Health Emergencies Program, told STAT.

The WHO said the risk at present to individuals in the general public appears to be low, but it may not stay that way; “immediate action from countries is required to control further spread among groups at risk, prevent spread to the general population and avert the establishment of monkeypox as a clinical condition and public health problem in currently non-endemic countries.”

Monkeypox is considered endemic in nature — though the actual animal host or hosts have not yet been identified — in roughly 12 countries in West and Central Africa.

There have been no reported fatalities in the current outbreak to date. And in fact, many of the infected people are reporting “relatively mild” symptoms, with swollen lymph nodes and lesions mainly in the mouth, or on or around the genitals or the anus, the WHO statement said. The lack of more serious illness may mean some affected people aren’t seeking medical care, which would contribute to the underestimation of the scale of the outbreak and hinder attempts to contain it.

The WHO also suggested health workers who aren’t familiar with monkeypox may not recognize what they’re seeing when an affected person comes in for care. “Due to the range of conditions that cause skin rashes and because clinical presentation may more often be atypical in this outbreak, it can be challenging to differentiate monkeypox solely based on the clinical presentation, particularly for cases with an atypical presentation,” it said.

It’s clear from the scale of the outbreak that the virus has been transmitting undetected for some time, and it is likely larger than is currently recognized. “[T]he sudden appearance and wide geographic scope of many sporadic cases indicates that widespread human-to-human transmission is already underway, and the virus may have been circulating unrecognized for several weeks or longer,” the agency said.

The WHO’s case definition for probable cases suggests health providers consider people who have had symptoms from March 15 onward as possible monkeypox cases, suggesting it believes the virus may have been transmitting undetected for at least that long.

The agency suggested countries should focus on communicating accurate information about monkeypox to groups currently at highest risk, and stopping further spread among them. It also said countries should work to protect front-line workers, saying health workers are at risk of contracting the virus if they are not using appropriate protective equipment.