The U.S. has nine confirmed cases of monkeypox in seven states, according to the CDC.
As of Thursday, there was one case in California; two cases in Florida; one case in Massachusetts; one case in New York; two cases in Utah; one case in Virginia and one case in Washington. CDC Director Dr. Rochelle Walensky said not everyone infected had traveled outside the U.S., which indicates community spread.
All of the U.S. cases so far have been in men who have sex with other men, Walensky said, but she warned that the virus is "not contained within social networks, and the risk of exposure is not limited to any one particular group."
As of Tuesday, there were more than 100 cases worldwide.
What is monkeypox?
Monkeypox is a virus that originates in wild animals like rodents and primates, and occasionally jumps to people. It belongs to the same virus family as smallpox.
Most human cases have been in central and west Africa and outbreaks have been relatively limited.
The illness was first identified by scientists in 1958 when there were two outbreaks of a "pox-like" disease in research monkeys — thus the name monkeypox. The first known human infection was in 1970, in a young boy in a remote part of Congo.
What caused the latest monkeypox outbreak?
Electron microscope image of various virions (virus particles) of the monkeypox virus taken from human skin, 2003. Courtesy CDC/Goldsmith. (Photo via Smith Collection/Gado/Getty Images)
Health officials are still investigating, but a top adviser to the World Health Organization said this week that the leading theory is that monkeypox was likely spread after sexual activity at two recent raves in Europe.
Dr. David Heymann, who chairs WHO's expert advisory group on infectious hazards, said monkeypox can spread when there is close contact with someone already infected with the disease, and that "it looks like sexual contact has now amplified that transmission."
Authorities in countries including Britain, Spain, Germany and Portugal say most of the known cases in Europe have been among men who have sex with men, but experts emphasize that anyone can be infected through close contact with a sick person, their clothing or bedsheets.
Scientists say it will be difficult to determine whether the spread is being driven by sex or merely close contact.
Why is it occurring more in men who have sex with men?
Close-up of a patient's hands, on black background, showing lesions from the monkeypox virus, the Democratic Republic of the Congo, 1997. Courtesy CDC/Mahey et al. (Photo via Smith Collection/Gado/Getty Images)
It's unclear, but the disease is no more likely to infect gay and bisexual men than anyone else.
"This may just be unlucky that (monkeypox) happened to get into this one particular community first," said Dr. Jake Dunning, an infectious diseases researcher at the University of Oxford, who is also involved in treating cases in London. "It's just that they are a community and by having sex with each other, that is how it's spreading," he said.
Other experts warn the disease could spread more widely if measures aren't taken to stop the outbreak.
"Infectious diseases don’t care about borders or social networks. Some groups may have a greater chance of exposure right now, but by no means is the current risk of exposure to monkeypox" exclusive to men who have sex with men, said the CDC’s Dr. John Brooks.
What are the symptoms of monkeypox?
Most monkeypox patients experience only fever, body aches, chills and fatigue. People with more serious illness may develop a rash and lesions on the face and hands that can spread to other parts of the body.
Most people recover within about two to four weeks without needing to be hospitalized, monkeypox can be fatal for up to 6% of cases and is thought to be more severe in children.
Smallpox vaccines are effective against monkeypox and anti-viral drugs are also being developed.
The European Centre for Disease Prevention and Control recommended all suspected cases be isolated and that high-risk contacts be offered a smallpox vaccine. The U.K. is offering high-risk contacts the smallpox vaccine and recommending anyone who might be infected to isolate until they recover.
The U.S. has 1,000 doses of a vaccine approved for the prevention of monkeypox and smallpox, plus more than 100 million doses of an older-generation smallpox vaccine in a government stockpile, officials said.
How worrying is this outbreak?
Any outbreak of an emerging virus is concerning, but most of the cases have been mild and there have been no deaths so far.
Monkeypox also requires very close contact to spread, so it is not likely to prompt big waves of disease like COVID-19, which can be transmitted in the air by people with no symptoms.
Still, Britain’s Health Security Agency has said it expects to see new infections "on a daily basis" and WHO’s Europe director warned that the summer season of festivals and parties could spread the disease. Many of the cases being identified have no links to previous infections, suggesting the virus is already spreading widely.
On Tuesday, Germany’s Health Minister Karl Lauterbach said the outbreak "is not the beginning of a new pandemic" but needed to be contained quickly.
How many monkeypox cases are there typically?
The World Health Organization estimates there are thousands of monkeypox infections in about a dozen African countries every year. Most are in Congo, which reports about 6,000 cases annually, and Nigeria, with about 3,000 cases a year.
In the past, isolated cases of monkeypox have been spotted outside Africa, including in the U.S. and Britain. The cases were mostly linked to travel in Africa or contact with animals from areas where the disease is more common.
In 2003, 47 people in six U.S. states had confirmed or probable cases. They caught the virus from pet prairie dogs that been housed near imported small mammals from Ghana.