Monkeypox (now officially called mpox) is caused by the monkeypox virus (MPXV), a DNA virus in the same family as smallpox and cowpox. The virus originally circulated in wild animals in Central and West Africa before jumping to humans, and it now spreads primarily through close skin-to-skin contact between people. As of June 2025, the WHO still classifies mpox as a public health emergency of international concern.
The Virus Behind Mpox
MPXV belongs to the Orthopoxvirus genus within the Poxviridae family. That makes it a relative of the variola virus (which causes smallpox), cowpox, and vaccinia (the virus used in smallpox vaccines). Unlike many viruses you hear about, MPXV carries double-stranded DNA rather than RNA, which makes it relatively large and stable compared to viruses like influenza or COVID-19.
Once the virus enters your body, it replicates inside your cells’ cytoplasm, not in the nucleus where your own DNA lives. It essentially builds tiny factories within your cells where it copies itself, assembles new viral particles, and then breaks out to infect neighboring cells. DNA synthesis kicks off within about two hours of infection at the cellular level, which is fast for a virus of this size.
Where the Virus Comes From
Despite the name, monkeys aren’t the main source. The virus was discovered in 1958 when outbreaks hit colonies of research monkeys, which is how it got its name. But the true natural reservoirs are small mammals in Central and West Africa, particularly rodents. Research comparing the geographic ranges of suspected host species points to Thomas’s rope squirrel as the most probable reservoir. Scientists have isolated the virus and fully sequenced its genome from this species, collected in what is now the Democratic Republic of the Congo.
Other arboreal rodents also show strong overlap with areas where the virus circulates, including red-legged sun squirrels and African dormice. A primate species, the sooty mangabey, has also tested positive. The first human case was recorded in 1970 in what is now the DRC, likely from direct contact with an infected animal.
Two Clades With Different Severity
Not all mpox is the same. The virus comes in two genetically distinct versions, called clades, that differ in where they circulate and how dangerous they are.
- Clade I originated in Central Africa and carries higher mortality rates, ranging from roughly 3% to 10% depending on the population and access to care. A newer sublineage called clade Ib has driven recent international concern due to sustained community transmission.
- Clade II is endemic to West Africa and is less severe. During outbreaks studied in Cameroon, the overall case fatality ratio for clade II was about 2.2%. This is the clade responsible for the large global outbreak that began in 2022.
Both clades cause the same general illness, but the difference in fatality rates is significant enough that public health agencies track them separately.
How It Spreads Between People
Mpox spreads primarily through close, direct contact. The most common routes are straightforward: touching the rash or scabs of someone who is infected, or coming into contact with their saliva, respiratory secretions, or other bodily fluids. Sexual and intimate contact is a major transmission pathway because it involves prolonged skin-to-skin exposure.
The virus can also survive on objects and surfaces. Clothing, bedding, towels, dishes, and shared personal items that haven’t been disinfected can carry the virus from one person to another. This is why household contacts of infected individuals face real risk even without direct physical contact with the rash itself.
One thing mpox does not appear to do well is spread through the air over distances. While some lab studies have detected the virus in respiratory droplets, and animal experiments have shown spread between closely housed animals, real-world outbreak data tells a different story. The disease hasn’t been shown to spread across rooms, in classrooms, in offices, or during travel the way airborne viruses do. Close proximity and direct contact remain the driving factors.
Incubation and Timing of Infection
After exposure, symptoms typically appear about 8 days later. A large pooled analysis of the 2022 global outbreak found the average incubation period was 8.1 days, which closely matches historical data going back decades (8.2 days). This gives the virus a relatively predictable window, though individual cases can fall outside that average.
The time between one person’s rash appearing and the next person developing a rash (called the serial interval) has historically averaged about 14.3 days. That gap was notably shorter during the 2022 outbreak, likely reflecting the close-contact transmission patterns seen in that wave. A person remains infectious as long as they have active skin lesions, which typically means several weeks from the first symptoms until all scabs have fallen off and new skin has formed underneath.
Why the Name Changed
In November 2022, the WHO officially adopted “mpox” as the preferred term, with a one-year transition period to phase out “monkeypox.” The change came after the 2022 global outbreak prompted racist and stigmatizing language tied to the old name. The WHO also noted that the original name, given in 1970, predated its own 2015 guidelines on disease naming, which recommend avoiding geographic or animal references that could harm trade, tourism, or specific communities. Both terms still appear in medical literature, but mpox is now the standard in official communications.