The World Health Organization on Wednesday declared the ongoing mpox outbreak in Africa a global health emergency, the highest level of alarm under international health law. It follows a public health emergency declaration that the Africa Centres for Disease Control and Prevention made for the continent Tuesday.

A deadlier strain of the virus, clade Ib, is spreading quickly in the Democratic Republic of Congo and has reached at least four previously unaffected countries in Africa. The risk of additional international spread is “very worrying,” WHO Director-General Tedros Adhanom Ghebreyesus said.

“It was unanimous that the current outbreak of mpox, upsurge of mpox, is an extraordinary event,” said Dimie Ogoina, chair of WHO’s emergency committee. “What we have in Africa is the tip of the iceberg. … We are not recognizing, or we don’t have the full picture of, this burden of mpox.”

Experts around the world are monitoring the situation closely. For now, the current risk to the United States is thought to be low. Here’s what to know.

Mpox, formerly known as monkeypox, is a viral disease related to the now-eradicated smallpox virus. It can spread through close contact such as touching, kissing or sex, as well as through contaminated materials like sheets, clothing and needles, according to WHO.

Initial symptoms are typically flu-like — including fever, chills, exhaustion, headache and muscle weakness — often followed by a painful or itchy rash with raised lesions that scab over and resolve over a period of weeks.

Mpox is characterized by two genetic clades, I and II. A clade is a broad grouping of viruses that has evolved over decades that has distinct genetic and clinical differences.

Clade II was responsible for a global outbreak that was also declared to be a global health emergency from July 2022 to May 2023.

But the new outbreak is driven by clade I, which causes more severe disease. The subtype that’s responsible for most of the ongoing spread, clade Ib, is relatively new.

“Because of a number of different factors, Ib has emerged as a new mutation that is adapted to humans,” said Dr. Daniel Bausch, senior adviser for global health security at FIND, a global nonprofit focused on health equity.

The virus is often transmitted from animals to humans, he said. But once a mutation adapts, it can be transmitted by humans and drive larger outbreaks.

Clade Ib is new and concerning, but the current situation is compounded by multiple overlapping outbreaks.

“We are not dealing with one outbreak of one clade; we are dealing with several outbreaks of different clades in different countries with different modes of transmission and different levels of risk,” Tedros said Wednesday.

Some outbreaks of clade I mpox have killed up to 10% of people who get sick, although more recent outbreaks have had lower death rates, according to the US Centers for Disease Control and Prevention. The fatality rate for clade II is less than 0.2%.

Certain groups – including infants, people with severely weakened immune systems and pregnant women – are more likely to have more severe infections.

Still, surveillance of mpox is quite incomplete, and there’s much more to learn, Bausch said.

“This is a virus that is in the environment and presumably maintained in small mammals in Africa, and we don’t have the proper diagnostics, really,” he said. “It’s not necessarily hard to diagnose mpox infection when you have a laboratory right next to you, and skilled laboratory workers, and the technology. But of course, most of these cases are often seen in very rural areas, so trying to get a sample and get it to a labora tory is a difficult thing.”

Our understanding of transmissibility and fatality risk may be skewed by limitations that tend to detect only the most severe cases, he said. Regardless, increased spread raises the public health burden and the number of individuals affected.

“It’s a disease that causes a lot of pain and suffering, a lot of fear, a lot of panic,” Bausch said.

For decades, mpox had largely been found in Central and West Africa. The vast majority of cases of clade I were from Central Africa and the Democratic Republic of Congo, and the vast majority of clade II were from Nigeria.

In 2022, concern grew when cases started spreading in Europe and in North America.

Broader international spread is a key reason why mpox has been declared a global health emergency, but the geographic pattern of the current outbreak is different than it was during the outbreak two years ago.

Now, mpox is reaching more countries in Africa that had previously been unaffected. While the vast majority of cases are still concentrated in the DRC, cases have also been reported in at least 13 countries across the continent, according to the Africa Centres for Disease Control and Prevention.

And on Thursday, Sweden confirmed its first case of clade I mpox, marking the first time it has been found outside Africa.

“We’ve all heard a million times that no one’s safe till everyone’s safe,” Bausch said, especially with how common international travel is. “There are many reasons to be concerned and jump on it early.”

There’s also an element of equity and human rights, he said. The global health emergency declaration from 2022 set a precedent, and similarly concerning patterns of transmission across multiple countries in Africa deserve as much urgency and attention as spread in high-income countries.

Vaccines to protect against mpox are available, but they’re not widely accessible in Africa.

No cases of clade I mpox have been identified in the United States, but the US CDC recommended last week that people in the US who are exposed to or at high risk of catching mpox should get vaccinated.

The Vaccine Alliance, known as Gavi, has up to $500 million to spend on supplying mpox vaccines to countries affected by the disease outbreak, including the Democratic Republic of Congo and surrounding countries. Beginning in 2026, Gavi will establish a global stockpile of mpox vaccines, similar to its existing stockpiles for cholera, Ebola, meningitis and yellow fever vaccines, a news release detailed Thursday.

But WHO emphasized that vaccines are only part of the response; containing the spread will also require increased surveillance, diagnostics and research to fill “gaps in understanding.”

The organization has signed off on the Emergency Use Listing process for both mpox vaccines and developed a regional response plan requiring $15 million, with $1.5 million released from the WHO Contingency Fund for Emergencies. Half a million doses of the vaccine are in stock, and another 2.4 million could possibly be produced by the end of the year, according to Tim Nguyen of the WHO Health Emergencies Program. The DRC and Nigeria will be the first to receive these vaccines, African Regional Emergency Director Dr. Abdou Salam Gueye added.

CNN’s Maya Davis and Caitlin Danaher contributed to this report.

Correction: This story has been updated to correct the fatality rate of clade II mpox.