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.

“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.

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.