The current monkeypox outbreak is by far the biggest involving the virus, recently designated as a global health emergency. So far, officials say, all evidence indicates that the disease has spread mainly through networks of men who have sex with men.
“It clearly is spreading as an STI (sexually transmitted infection) at this point,” said Dr. Tom Inglesby, director of the Johns Hopkins Center for Health Security.
To protect the people at highest risk while trying to contain the spread, public health agencies are focusing their attention on those men — and attacking the virus based on how it's behaving now.
On Wednesday, the head of the World Health Organization advised men at risk for monkeypox to consider reducing their sexual partners “for the moment.”
But this is a complicated outbreak that may shift in how it spreads and which population groups are most affected.
There is also debate about whether monkeypox should be called a sexually transmitted disease, with some critics complaining that the term creates a stigma and could be used to vilify gay and bisexual men.
Monkeypox can spread in nonsexual ways too, and it's not enough to use condoms or other typical measures for stopping STDs, experts say.
But in May, cases began emerging in Europe, the United States and elsewhere that showed a clear pattern of infection through intimate contact with an infected person, like many other sexually transmitted diseases.
WHO officials said last week that 99% of all the monkeypox cases beyond Africa were in men and that of those, 98% involved men who have sex with men. Experts suspect that monkeypox outbreaks in Europe and North America were ignited by sex at two raves in Belgium and Spain.
The statistics are the same for cases reported in the United States, according to the Centers for Disease Control and Prevention. As in Europe, cases have emerged in other groups too, including at least 13 people who were female at birth and at least two children.
The virus has not usually spread easily among people, and experts are still trying to understand exactly how it moves from person to person. In Africa, where small outbreaks have been common for years, people have been infected through bites from rodents or small animals.
Some researchers have found evidence of the monkeypox virus in semen. A study in Spain found monkeypox virus DNA in the semen of some infected men, as well as in saliva and other body fluids. But the study didn't answer whether the virus actually has spread through semen.
Meanwhile, scientists believe the primary route of transmission during the current outbreak has been skin-to-skin contact during sexual encounters with someone who has symptoms. In that respect, it's similar to herpes, some experts noted.
The virus also may spread through saliva and respiratory droplets during prolonged, face-to-face contact, such as during kissing and cuddling — a kind of spread that can occur outside of sex.
Officials also say people can catch monkeypox from touching items that previously touched an infected person’s rash or body fluids, such as towels or bedsheets. That is thought to explain the infections of the U.S. children.
The U.S. government has been shipping a monkeypox vaccine, but the supply is limited. So far it’s only been recommended as a post-exposure treatment or for people who have had multiple sex partners in the past two weeks in a place where monkeypox cases have been reported.
The vaccine is new, and officials are trying to gather data on exactly how well it works.