Poxviruses are a family of large, double-stranded DNA viruses responsible for diseases ranging from the now-eradicated smallpox to the recently resurgent mpox, along with less familiar infections like molluscum contagiosum and orf. They spread primarily through direct contact with infected skin, bodily fluids, or contaminated materials, and some can jump between animals and humans. Prevention relies on a combination of vaccines originally developed against smallpox, surveillance, and basic infection control, but the picture has grown more complicated since routine smallpox vaccination stopped decades ago.
What Makes Poxviruses Unusual
Most DNA viruses need to get inside the cell’s nucleus to copy themselves, piggybacking on the host’s own machinery for reading and duplicating genetic material. Poxviruses skip that step entirely. They replicate in the cytoplasm, the main body of the cell outside the nucleus, and they carry their own toolkit for the job. Their genomes encode the enzymes needed for both DNA replication and the production of messenger RNA, making them remarkably self-sufficient compared to other viruses.1PubMed. Structure and function of the poxvirus transcription machinery That independence comes at a cost in size: poxvirus particles are among the largest of any virus, big enough to be faintly visible under a good light microscope.
Their structure matters for how they spread. The well-studied vaccinia virus, a close relative of the smallpox virus, produces two distinct infectious forms. One stays inside the cell until it bursts open, releasing particles that are tough and environmentally stable. The other acquires an extra membrane wrapping as it exits the cell, giving it a different set of surface proteins and the ability to bind to different receptors on new target cells.2PubMed Central. The extracellular domain of vaccinia virus protein B5R affects plaque phenotype, extracellular enveloped virus release, and intracellular actin tail formation This dual strategy helps the virus spread locally within the body and also move between hosts. The enveloped form slips into cells without triggering the same alarm bells that the simpler form does, essentially entering quietly.3PubMed Central. Entry of the two infectious forms of vaccinia virus at the plasma membane is signaling-dependent for the IMV but not the EEV
Diseases Caused by Poxviruses
The poxvirus family is large, and only a handful of its members regularly infect humans. But the ones that do range from mild nuisances to historically devastating killers.
Smallpox
Smallpox, caused by variola virus, killed an estimated 300 million people in the twentieth century alone before a global vaccination campaign declared it eradicated in 1980. The effort combined mass vaccination in wealthier countries with a strategy called surveillance and containment in developing regions, where health workers conducted house-to-house searches, offered rewards for reported cases, and vaccinated everyone around each new infection to create a firebreak.4PubMed. The global eradication of smallpox Variola virus no longer circulates in nature, but official stocks remain in two high-security laboratories, and the virus’s potential as a bioweapon keeps it on the radar of public health agencies worldwide.
Mpox
Mpox, formerly called monkeypox, has become the most prominent poxvirus threat in the post-smallpox era. The virus comes in distinct genetic lineages, or clades, that differ sharply in how dangerous they are. Clade I, found primarily in Central Africa, carries a case fatality rate of roughly 5 to 10 percent. Clade II, which includes the sublineages responsible for the 2022–2024 global outbreak, is less deadly but spreads more easily, especially through close skin-to-skin and intimate contact.5PubMed Central. Comparative analysis of Mpox clades: epidemiology, transmission dynamics, and detection strategies Animal studies have confirmed these differences in virulence: in lab models, clade I virus was lethal at extremely low doses, while even very high doses of the clade IIb.1 virus behind recent outbreaks did not kill any animals tested.6PubMed Central. Virulence differences of mpox (monkeypox) virus clades I, IIa, and IIb.1 in a small animal model Still, clade II infections can produce painful lesions, and hospitalization is sometimes necessary.
Other Human Poxvirus Infections
Molluscum contagiosum is probably the most common poxvirus infection people actually encounter. It produces small, firm, dome-shaped bumps on the skin and spreads through direct contact, including sexual contact in adults and casual skin contact among children. It is generally harmless and resolves on its own, though it can be stubborn in people with weakened immune systems.
Orf virus causes a different kind of skin lesion. It primarily infects sheep, goats, cattle, and deer, and people pick it up through direct contact with infected animals or contaminated meat.7PubMed. Orf, a Human Parapoxvirus Infection Cases tend to cluster around occupational exposure on farms and slaughterhouses, though children have also been infected at petting zoos and livestock fairs.8PubMed. Human Orf virus infection from household exposures – United States, 2009-2011 The resulting sore usually heals without treatment in a few weeks.
Poxviruses in Animals
Humans are not the only ones dealing with poxvirus problems. Cowpox virus circulates in wild rodent populations across Europe, with bank voles and wood mice serving as its main reservoir hosts.9PubMed Central. A longitudinal study of an endemic disease in its wildlife reservoir: cowpox and wild rodents From those rodents, the virus can spill over into cats, zoo animals, and occasionally humans. Despite its name, cowpox rarely infects cows today.
Lumpy skin disease, caused by a capripoxvirus, is a serious veterinary concern in Africa and has spread into parts of Europe and Asia. Infected cattle develop widespread nodules across the skin, along with fever, weight loss, and reduced milk production. The skin damage is permanent, ruining hides and causing lasting economic losses for herders.10PubMed. Lumpy skin disease, an African capripox virus disease of cattle The disease does not infect humans, but its economic impact in affected regions is severe, and it spreads readily through biting insects.
How Poxviruses Spread
Transmission routes vary across the family, but a few patterns are consistent. Direct contact with skin lesions or the fluid they contain is the most efficient way for many poxviruses to move between hosts. For mpox, the 2022–2024 outbreaks made clear that prolonged skin-to-skin contact, including during sexual activity, was the dominant route of spread outside endemic regions.5PubMed Central. Comparative analysis of Mpox clades: epidemiology, transmission dynamics, and detection strategies Respiratory transmission is possible for some orthopoxviruses, particularly smallpox, which spread efficiently through respiratory droplets in crowded settings. Mpox can also spread through respiratory secretions during prolonged face-to-face contact, though this appears less common than skin contact in recent outbreaks.
Contaminated surfaces, or fomites, add another dimension. Mpox virus can survive on smooth, nonporous materials for days. On glass and stainless steel, viable virus has been detected for up to five days at room temperature, and on plastic for up to three days. Porous surfaces like wood and cardboard are far less hospitable, with the virus becoming undetectable after a day or two.11PubMed. Stability of mpox virus on different commonly contacted surfaces At cold temperatures the picture changes considerably: on nonporous materials like medical tubing and gloves kept at 4°C, viable mpox virus has been recovered for up to three weeks.12Open Forum Infectious Diseases. Stability of Monkeypox Virus on Commonly Contacted Surfaces in Clinical Settings At room temperature, viral counts on those same surfaces dropped sharply after the first day. The practical takeaway is that regular cleaning and disinfection of shared surfaces matter, especially in clinical environments where temperatures may be lower and contact with medical equipment is frequent.
How Poxviruses Evade the Immune System
One reason poxviruses can cause serious illness is that they have evolved an elaborate set of tools for dodging and disabling the host’s immune defenses. The immune system does recognize poxvirus infections and mounts responses against them, but the viruses can actively suppress those responses.13PubMed Central. Poxviruses and the immune system: Implications for monkeypox virus
A standout trick is the production of decoy molecules. Poxviruses secrete proteins that mimic the host’s own cytokines and cytokine receptors, the signaling molecules the immune system uses to coordinate its attack. These decoys intercept immune signals before they reach their targets, effectively blinding parts of the defense.14PubMed. Poxvirus Immune Evasion Some poxviruses go further by producing a fake version of the gamma interferon receptor, one of the most important antiviral signaling molecules. Research on ectromelia virus, which causes mousepox, showed that when this decoy receptor was deleted from the virus, infected animals mounted a much stronger immune response and were more likely to survive. Whether an animal lived or died came down to the balance between its ability to produce interferon gamma and the virus’s ability to neutralize it.15PubMed Central. Poxvirus-encoded gamma interferon binding protein dampens the host immune response to infection
Poxviruses also target the complement system, a network of proteins that punches holes in invaders and flags them for destruction. Vaccinia virus produces a protein that structurally mimics human complement regulators and actively breaks down key complement components, slowing the cascade before it can damage the virus.16PubMed. Domain swapping reveals complement control protein modules critical for imparting cofactor and decay-accelerating activities in vaccinia virus complement control protein The sheer number of immune evasion genes poxviruses carry, often dozens per genome, reflects millions of years of evolutionary arms-racing with mammalian hosts.
Vaccines and Cross-Protection
The original smallpox vaccine, based on vaccinia virus, remains the foundation of poxvirus prevention. Because orthopoxviruses are closely related, vaccination against one tends to generate immunity that works against others. This cross-protection is central to the current mpox response: a meta-analysis pooling data from multiple studies found that smallpox-vaccinated individuals had roughly half the risk of mpox infection compared to unvaccinated people, and among those who did get infected, the risk of severe disease was also reduced.17PubMed Central. Global perspectives on smallpox vaccine against monkeypox: a comprehensive meta-analysis and systematic review of effectiveness, protection, safety and cross-immunogenicity That protection can last for decades. A study of people aged 50 and older in Spain, all vaccinated against smallpox in childhood, found that about two-thirds still had detectable antibodies against vaccinia virus.18PubMed Central. Possible Mpox Protection from Smallpox Vaccine-Generated Antibodies among Older Adults
The newer-generation vaccine in wide use today is JYNNEOS (also known as MVA-BN or Imvanex), a modified vaccinia virus that cannot replicate in human cells and therefore has a much milder side effect profile than the older live vaccines. In the United States during the 2022 outbreak, two doses of JYNNEOS showed an estimated effectiveness of about 66 percent against mpox, while a single dose provided around 36 percent protection.19PubMed Central. Vaccine Effectiveness of JYNNEOS against Mpox Disease in the United States A broader systematic review of real-world outbreak data reported effectiveness ranging from 35 to 86 percent after one dose and 66 to 90 percent after two doses, depending on the study setting.20PubMed. MVA-BN vaccine effectiveness: A systematic review of real-world evidence in outbreak settings Third-generation vaccines like JYNNEOS also showed greater efficacy than older first-generation formulations in the meta-analysis data.17PubMed Central. Global perspectives on smallpox vaccine against monkeypox: a comprehensive meta-analysis and systematic review of effectiveness, protection, safety and cross-immunogenicity Two doses clearly perform better than one, which matters because early in the 2022 response many people received only a single shot due to supply constraints.
Why Mpox Keeps Resurging
The emergence of mpox as a global concern is tightly linked to the eradication of smallpox itself. When routine smallpox vaccination ended in the late 1970s and early 1980s, the population-wide immunity it had been providing against all orthopoxviruses began to fade. Younger generations born after that cutoff have never been vaccinated and have no cross-protective antibodies.21PubMed. Emergence of mpox in the post-smallpox era-a narrative review on mpox epidemiology This growing immunity gap is the single biggest factor behind the changing patterns of mpox outbreaks.
The consequences go beyond more infections. With sustained human-to-human transmission now occurring widely for the first time, the virus is accumulating genetic mutations at a faster rate than when it mostly circulated in animal reservoirs with only occasional jumps to humans. Some of those mutations already affect the accuracy of certain diagnostic tests, and there is concern that continued adaptation could eventually change how virulent the virus is.22PubMed Central. The Rise of Mpox in a Post-Smallpox World Meanwhile, the clade I epidemic in the Democratic Republic of Congo continues, and low population immunity across much of the world means the raw ingredients for further outbreaks are in place.23Global Biosecurity. Mpox, smallpox and the increasing threat of orthopoxvirus epidemics
Treatment and Antivirals
For most poxvirus infections, treatment is supportive: managing pain, keeping lesions clean, and preventing secondary bacterial infections. But for severe cases of mpox or for patients at high risk of complications, an antiviral drug called tecovirimat (brand name TPOXX) is available. Tecovirimat was originally developed and stockpiled as a countermeasure against a potential smallpox attack. It works by blocking a viral protein called F13 that the virus needs to wrap its infectious particles in an outer membrane and release them from infected cells. Without that wrapping step, the virus can still replicate inside cells but cannot spread efficiently to new ones.
Recent structural work has shown that tecovirimat acts as a kind of molecular glue, forcing two copies of the F13 protein to stick together in a way that disables the protein’s normal function. Some mpox clinical isolates have developed mutations at the spot where those two copies meet, preventing the drug from gluing them together and thereby conferring resistance.24PubMed Central. Mechanisms of tecovirimat antiviral activity and poxvirus resistance Resistance is still uncommon, but its emergence has prompted discussions about developing additional antivirals and using combination therapy rather than relying on a single drug.
Diagnosing Poxvirus Infections
Telling poxviruses apart clinically can be tricky. Mpox, chickenpox (which is not a poxvirus at all, but a herpesvirus), and even syphilis can produce skin lesions that look superficially similar. Laboratory confirmation is essential for public health responses. The primary tool is PCR testing on material swabbed from skin lesions, which can detect and differentiate between orthopoxvirus species based on characteristic genetic signatures.25PubMed. Sequence alterations within and downstream of the A-type inclusion protein genes allow differentiation of Orthopoxvirus species by polymerase chain reaction During the 2022 mpox outbreak, diagnostic capacity scaled up rapidly in many countries, but the ongoing genetic evolution of the virus is an emerging concern for test accuracy, as mutations in the target regions could cause some assays to miss infections.22PubMed Central. The Rise of Mpox in a Post-Smallpox World
Biosecurity and Synthetic Biology
The eradication of smallpox was one of humanity’s greatest public health achievements, but it left behind a peculiar vulnerability. With vaccination stopped and natural immunity fading, a re-introduction of variola virus into the global population would find a world with very little defense. The development of synthetic biology has made that scenario less theoretical than it used to be. In 2017, researchers demonstrated the first de novo synthesis of horsepox virus, an extinct orthopoxvirus closely related to smallpox, assembling it from commercially available synthetic DNA fragments.26PubMed. The De Novo Synthesis of Horsepox Virus: Implications for Biosecurity and Recommendations for Preventing the Reemergence of Smallpox
The immediate concern was not that horsepox itself is dangerous but that the same techniques could be applied to variola. The synthesis demonstrated that two of the main barriers to smallpox’s return, the difficulty of obtaining the virus and the specialized knowledge required to work with it, were both eroding. If orthopoxvirus synthesis becomes routine for legitimate research and vaccine development, the expertise and infrastructure needed to create variola would spread along with it.27PubMed Central. A Holistic Assessment of the Risks and Benefits of the Synthesis of Horsepox Virus This has led to calls for stronger international oversight of DNA synthesis orders and tighter controls on who can work with orthopoxvirus sequences. The debate remains unresolved: supporters of the horsepox work argue it could lead to improved vaccines, while critics contend the biosecurity risks outweigh the benefits when existing vaccines already work well.
In the background, the continued circulation of mpox and the possibility of other orthopoxviruses adapting more efficiently to human-to-human spread serve as a natural reminder that poxviruses remain an active threat, not just a historical one. Maintaining vaccine stockpiles, investing in next-generation antivirals, and strengthening surveillance networks are all part of keeping that threat manageable.