Monkeypox, now officially called mpox, originated in animals in Central and West Africa. Despite its name, the virus wasn’t first found in wild monkeys. It was discovered in 1958 during an outbreak among captive monkeys at a research facility in Copenhagen, Denmark, and the misleading name stuck. The actual natural reservoirs are most likely small rodents, particularly squirrels, living in tropical African forests.
How the Virus Was First Discovered
In 1958, a group of crab-eating macaques arrived at a Danish animal facility from Singapore for use in polio vaccine research. About two months after the monkeys arrived, six out of 32 animals broke out in blistering skin eruptions covering their trunks, tails, faces, limbs, palms, and the soles of their feet. Researchers isolated a previously unknown virus from the pustules, and because monkeys were the first animals they observed it in, they named it “monkeypox virus.”
The monkeys actually recovered within about two weeks. Their lesions crusted over, healed, and fell off, leaving scars. The disease appeared relatively mild in these animals, but the virus itself turned out to be a close relative of smallpox, belonging to the same family of orthopoxviruses. The first known human case wouldn’t be documented until 1970, in the Democratic Republic of the Congo (DRC).
The Real Animal Source: African Rodents
Monkeys can catch mpox, but they aren’t where the virus naturally lives. The true reservoirs are small mammals in Central and West Africa, and the strongest evidence points to tree-dwelling rodents. Ecological studies have found that the four mammal species whose habitats overlap most closely with known virus circulation are all arboreal rodents: Thomas’s rope squirrel, red-legged sun squirrel, red-legged rope squirrel, and a species of African dormouse.
Scientists have fully sequenced the virus from two wild animal species: Thomas’s rope squirrel, captured in the DRC’s Mongala Province, and the sooty mangabey, a primate from Côte d’Ivoire’s Taï National Park. But the broader pattern of evidence, including habitat modeling and antibody surveys, consistently points to squirrels and other small rodents as the primary hosts that maintain the virus in the wild. The virus likely spills over into primates (including humans) when they come into contact with these rodents.
How the Virus Jumps to Humans
People catch mpox from animals through direct contact. The World Health Organization identifies several routes: bites or scratches from infected animals, or handling them during hunting, skinning, trapping, cooking, or eating. Children playing with animal carcasses in endemic regions is another documented pathway. Contact with an infected animal’s blood, bodily fluids, or waste can also transmit the virus.
Once a person is infected, they can spread it to other people through close physical contact with skin lesions, respiratory droplets during prolonged face-to-face interaction, or contaminated materials like bedding. The incubation period averages about eight days, after which fever, swollen lymph nodes, and flu-like symptoms typically appear. The characteristic rash follows, starting as flat spots that progress through raised bumps, fluid-filled blisters, and pus-filled pustules before crusting over and healing.
Two Clades From Two Regions
The virus exists in two genetically distinct versions, or clades, that evolved separately in different parts of Africa. Clade I circulates in Central Africa, centered on the DRC and neighboring countries. Clade II is endemic to West Africa. This geographic split has been documented in Cameroon, which sits at the boundary between the two regions and has recorded both clades circulating within its borders since the late 1970s.
The two clades differ in severity. Clade I has historically caused more serious illness, with fatality rates reported between 1% and 10% depending on the outbreak and the population affected. Clade II is generally milder. In one long-running analysis of cases in Cameroon, the case-fatality ratio for Clade II was 2.2%.
The global outbreak that began in May 2022 was driven by Clade IIb, the less severe variant, which spread primarily through close physical contact. That outbreak marked the first time mpox had sustained widespread human-to-human transmission outside Africa.
The Clade Ib Variant and Recent Concerns
In 2024, a new strain called Clade Ib emerged in the DRC and quickly spread to neighboring countries that had never previously reported mpox cases. On August 14, 2024, the WHO declared this upsurge a public health emergency of international concern.
Clade Ib is concerning for two reasons. First, like the Clade IIb strain behind the 2022 global outbreak, it shows sustained human-to-human transmission rather than relying on repeated animal-to-human spillover events. Second, laboratory studies suggest it’s more dangerous. In animal models, Clade Ib caused significant weight loss and high mortality, with more severe lung damage and wider spread to internal organs compared to Clade IIb. Researchers noted that unlike Clade IIb, which appeared to become less virulent as it adapted to human transmission, Clade Ib has not undergone the same attenuation.
Why the Name Changed to Mpox
In November 2022, the WHO formally recommended replacing “monkeypox” with “mpox.” The name change had two motivations. Practically, the original name was a misnomer: monkeys are incidental hosts, not the source. More urgently, during the 2022 outbreak, the WHO documented racist and stigmatizing language being directed at communities associated with the disease, partly fueled by the name itself. Under the WHO’s 2015 naming guidelines, disease names should avoid causing harm to trade, travel, animal welfare, or any cultural, social, or ethnic group. The 1970 name predated those guidelines by decades. Both “monkeypox” and “mpox” remain in use, but official communications now favor mpox.