Sweden has just announced the country’s first case of clade 1 mpox, a deadlier version of mpox that previously had been reported only in Africa.Mpox, formerly known as monkeypox, is caused by a group of viruses with two main branches in their family tree: clade 1 and clade 2. The latter caused the global mpox emergency in 2022 and 2023, which affected countries all over the world, including the U.S., the U.K. and Sweden. In comparison, clade 1 mpox viruses cause more severe disease and death and had never been seen outside Africa — until now.On Thursday (Aug. 15), the Public Health Agency of Sweden announced the detection of a clade 1 virus in an individual who sought medical treatment in Stockholm. The person was likely infected “during a stay in the part of Africa where there is a major outbreak of mpox clade I,” the agency reported.Clade 1 viruses are a major contributor to ongoing mpox outbreaks in the Democratic Republic of the Congo (DRC) and other African nations, including Burundi, the Central African Republic and the Republic of Congo. The World Health Organization (WHO) declared these outbreaks a public health emergency of international concern on Wednesday (Aug. 14), flagging a fairly new clade 1 virus as being of particular concern.Related: Mpox cases are far outpacing last year’s numbers, CDC reportsScientists first detected this form of the virus, dubbed clade 1b, in the DRC. It has since sparked infections in African countries where mpox had never been reported, such as Burundi, Kenya, Rwanda and Uganda.The Public Health Agency of Sweden did not clarify which subtype of clade 1 virus was detected in their case. Clade 1b is estimated to have a case-fatality rate between 3% and 6%, while in general, clade 1 viruses have reported rates up to 10%. By comparison, in the 2022-2023 outbreak, clade 2 viruses had fatality rates as as low as 0.2%Get the world’s most fascinating discoveries delivered straight to your inbox.”The fact that a patient with mpox is treated in the country does not affect the risk to the general population, a risk that the European Centre for Disease Prevention and Control (ECDC) currently considers very low,” the agency emphasized. “A new assessment is expected shortly. However, occasional imported cases like the current one may continue to occur.”If needed, Sweden can use antiviral drugs that are sometimes used to treat mpox; no drugs are approved to treat mpox, specifically, but drugs designed for similar viruses, such as the virus behind smallpox, can help.Mpox vaccines can also be given to help prevent illness following a known or suspected exposure to the virus. In what’s known as a “ring” vaccination strategy, vaccines can be given to the close contacts of an infected person to prevent outbreaks from widening. Sweden’s health agency did not note whether it might employ such a strategy.”This case does not require any additional infection control measures in itself, but we take the outbreak of clade I mpox very seriously,” Magnus Gisslén, the state epidemiologist for the Public Health Agency of Sweden, said in the statement. “We are closely monitoring the outbreak and we are continuously assessing whether new measures are needed.”Previously, Sweden reported about 300 cases of clade 2 mpox infections within the country, all connected to the 2022-2023 global outbreak.Ever wonder why some people build muscle more easily than others or why freckles come out in the sun? Send us your questions about how the human body works to community@livescience.com with the subject line “Health Desk Q,” and you may see your question answered on the website!