With a severe version of mpox breaking out in the Democratic Republic of Congo, officials with the U.S. Centers for Disease Control and Prevention are warning that it could spread and that people at risk should be vaccinated as soon as possible.

High-risk populations include men who have sex with men, although anyone can contract the disease, which is largely spread by skin-to-skin contact — sexual relations, for instance, but also in crowds at outdoor festivals. Thousands of gay and bisexual men developed mpox in a 2022 outbreak. People with compromised immune systems, such as those living with HIV, are at high risk for serious cases and death.

“There are two main types of mpox: Clade I, the type that is dominant in Congo, and Clade II, a version of which caused the 2022 global outbreak,” The New York Times reports. Clade I is deadlier than Clade II, the Times notes, explaining that “a clade is a genetically and clinically distinct group of viruses.”

No Clade I cases have been documented outside of Congo yet, but a global spread is possible, as the 2022 outbreak that affected Europe and North America originated in Nigeria.

“This is a very important example of how an infection anywhere is potentially an infection everywhere, and why we need to continue to improve disease surveillance globally,” Anne Rimoin, an epidemiologist at the University of California, Los Angeles, told the Times.

Many of the cases in Congo were among sex workers, both male and female, and their clients, plus men who have sex with men, according to the CDC. The symptoms of mpox infection include fever, severe headache, back pain, a rash, and sometimes sores at the infection site.

There were about 30,000 cases of mpox in the U.S. in 2022 but only 1,700 last year, thanks to vaccines and changes in behavior, the CDC reports. However, only 23 percent of at-risk Americans have gotten both necessary doses of the Jynneos vaccine against mpox, so CDC scientists are urging all those at risk to be vaccinated. The vaccine is now available at pharmacies in the U.S., making it easier to access than previously.

A recent Government Accountability Office report found serious flaws in the federal government’s response to the 2022 outbreak. The Department of Health and Human Services’ internal communication was poor, hampering coordination of efforts to fight mpox, according to the report. Democratic U.S. Rep. Ritchie Torres of New York, a gay man, has introduced legislation to improve communication within federal agencies and messaging to at-risk populations.

“The federal government and HHS were catastrophically unprepared for the mpox outbreak,” Torres told The Advocatein April.