The World Health Organization (WHO) says nearly 200 cases of monkeypox have been tallied globally.
The cases have been found in more than 20 countries not usually known to have outbreaks of the virus.
The agency still described the reported monkeypox outbreaks as "containable," although there are still many unknowns about what triggered the infections and the reported cases are likely an undercount.
Dr. Sylvie Briand, WHO’s director of pandemic and epidemic diseases, explained that numbers may increase in the coming days and that the event is "unusual."
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She said during a Friday media briefing that the spread was likely due to a change in "human behavior" instead of in the virus, although the WHO is still investigating the cause.
"There are also many uncertainties about the future and this disease … because we don't know if this transmission will stop," she said, noting that the WHO hopes there will be "self-limiting outbreaks" and that they do not know the extent of the disease.
Currently, Briand added, they don't know if the world is just seeing the peak of the iceberg and that it's hard to assess risk in communities.
Traditionally, monkeypox virus is spread by touching or getting bitten by infected wild animals in western and central Africa.
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However, the former World Health Organization's (WHO) emergencies department leader told The Associated Press earlier this week that cases in Europe appear to have spread due to sexual activity at raves in Spain and Belgium.
While it is not a sexually transmitted infection, it can be transmitted in both personal and sexual contact, with a notable fraction of recorded cases occurring among gay and bisexual men.
However, Spanish authorities said Friday that cases there had risen to 98, including a woman whose infection is "directly related" to a chain of transmission that had been previously limited to men.
Caseloads in Portugal and the U.K. also rose.
To treat monkeypox, some smallpox vaccines and therapeutics are available – no vaccines have been specifically developed against monkeypox – and the WHO proposed creating a stockpile to equitably share what was available.
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Dr. Rosamund Lewis, head of WHO’s smallpox department, said that "there is no need for mass vaccination," because monkeypox does not spread easily.
Data from the Centers for Disease Control and Prevention (CDC) shows there are 10 cases across eight U.S. states.
The Associated Press contributed to this report.