Poxviruses are among the largest and most complex viruses known to infect animals and humans, carrying double-stranded DNA genomes big enough to encode their own transcription machinery. That self-sufficiency sets them apart from most other DNA viruses, which depend on the host cell’s nucleus to copy and read their genetic instructions. The family includes the virus behind smallpox, the only human disease ever eradicated by vaccination, as well as mpox, molluscum contagiosum, and a growing list of animal pathogens that occasionally jump to people. Understanding how these viruses are built, how they spread, and what makes them medically important requires looking well beyond any single disease.
What Makes a Poxvirus a Poxvirus
Poxviruses are big by viral standards. Their genomes range from roughly 122 kilobases in the smallest known member (cetacean poxvirus) to about 457 kilobases in carp edema virus, a nearly four-fold span across the family.1bioRxiv. Evolutionary Variation of Poxvirus Genome Architecture Most human-relevant poxviruses fall somewhere in the middle, with genomes around 180 to 230 kilobases. That is enormous compared with something like influenza, whose genome fits into about 13 kilobases.
All that genetic real estate allows poxviruses to pack in genes for their own RNA polymerase, capping enzymes, and other transcription factors. Their RNA polymerase is evolutionarily related to the host cell’s own version but works independently in the cytoplasm, the watery interior of the cell outside the nucleus.2PubMed. Cytoplasmic gene expression: lessons from poxviruses This is unusual. Most DNA viruses need to get into the nucleus to borrow the cell’s copying machinery. Poxviruses skip that step entirely, setting up dedicated “factories” in the cytoplasm where both transcription and translation happen in close quarters.3PubMed Central. Colocalization of transcription and translation within cytoplasmic poxvirus factories coordinates viral expression and subjugates host functions
Under an electron microscope, poxvirus particles look brick-shaped or oval, depending on the species. The mature form of vaccinia virus, the best-studied poxvirus, is surrounded by a single membrane. A second form, wrapped in an extra lipid envelope carrying additional viral proteins, is the version that helps the virus spread between cells and between hosts.4PubMed. Vaccinia virus morphogenesis and dissemination These two particle types play different roles in infection: the single-membrane form is tough and stable in the environment, while the double-wrapped form is better at evading antibodies and moving through the bloodstream.
The Two Big Branches of the Family
Poxviruses split into two subfamilies. The Chordopoxvirinae infect vertebrates, from humans and cattle to reptiles and birds. The Entomopoxvirinae infect insects.5PubMed Central. New insights into the evolution of Entomopoxvirinae from the complete genome sequences of four entomopoxviruses infecting Adoxophyes honmai, Choristoneura biennis, Choristoneura rosaceana, and Mythimna separata Most of the viruses people worry about sit in the vertebrate branch, specifically within the genus Orthopoxvirus. That genus includes variola (smallpox), mpox virus, vaccinia, cowpox, and camelpox. But the vertebrate branch is far broader than orthopoxviruses alone: it also houses parapoxviruses like orf virus, molluscipoxvirus (molluscum contagiosum), and avipoxviruses that cause disease in birds.
As poxvirus genomes have grown larger across evolutionary time, their “coding density” has gone down slightly, meaning a bigger fraction of the genome appears to be non-coding sequence between genes.1bioRxiv. Evolutionary Variation of Poxvirus Genome Architecture What those non-coding stretches do, if anything, is still an open question. Some researchers suspect they harbor regulatory elements; others think much of it may simply be genomic baggage accumulated over long evolutionary timescales.
Diseases in Humans
The most consequential human poxvirus was variola, the agent of smallpox. Before eradication, smallpox killed an estimated 30 percent of the people it infected in its severe form (variola major) and produced disfiguring scars in survivors. Routine vaccination and a targeted strategy of identifying cases and vaccinating their contacts, known as ring vaccination, drove the virus to extinction in the wild by 1980.6PubMed Central. Ring vaccination and smallpox control Today, variola virus exists only in two high-security laboratories, one in the United States and one in Russia.
Mpox virus, formerly called monkeypox virus, has filled part of the ecological space that smallpox left. It circulates in rodents in Central and West Africa, and it spills over into humans through contact with infected animals or other people. The virus comes in distinct genetic groups, called clades, with meaningfully different severity. In animal models, clade I causes the most severe disease, clade IIa is intermediate, and clade IIb.1, the strain behind the 2022 global outbreak, is the mildest. Mice infected with clade IIb.1 showed no deaths even at doses a hundred times higher than the lethal dose of a closely related clade IIa virus.7PubMed Central. Virulence differences of mpox (monkeypox) virus clades I, IIa, and IIb.1 in a small animal model A newer lineage, clade Ib, which emerged in the Democratic Republic of the Congo, appears more virulent again. In the same mouse model, clade Ib caused severe weight loss and death in nearly all animals, with worse lung damage and wider spread to internal organs compared with clade IIb.8Communications Biology. Enhanced virulence of mpox virus clade Ib over clade IIb in the CAST/EiJ mouse model This matters because clade Ib is now spreading in parts of Africa and has been detected in travelers elsewhere.
Molluscum contagiosum is the most common poxvirus infection in everyday clinical practice, producing small, dome-shaped, waxy bumps on the skin. It mainly affects children, sexually active young adults, and people with weakened immune systems. The infection resolves on its own, though that spontaneous clearance can take months or even years.9PubMed Central. Molluscum Contagiosum: Epidemiology, Considerations, Treatment Options, and Therapeutic Gaps It spreads through direct skin contact or shared items like towels, and it is not dangerous, but it can be cosmetically distressing and persistent enough to drive people to seek treatment.
Zoonotic Poxviruses and How They Jump to People
Several poxviruses live primarily in animals but occasionally cross into humans. Orf virus, a parapoxvirus, is a good example. It normally causes contagious ecthyma, a mouth and skin disease in sheep and goats. People who handle infected animals, especially farmers and veterinarians, can develop painful nodular lesions on their hands, usually through a cut or abrasion.10PubMed. Orf virus infection in sheep or goats Transmission can also happen indirectly through contaminated fomites like fences, feeding troughs, and shearing equipment.11PubMed Central. A Review on Human Orf: A Neglected Viral Zoonosis Orf lesions in humans are self-limiting but can be alarming enough to trigger concern about more dangerous infections.
Cowpox, despite its name, now circulates mainly in rodents across Europe rather than in cattle. Human infections are rare and usually mild. More recently, a novel orthopoxvirus was isolated from a resident of Alaska in 2015. Phylogenetic analysis showed it was a distinct species, now called borealpox virus (initially named Alaskapox virus), more closely related to Old World orthopoxviruses than to the raccoonpox and skunkpox viruses found in North America.12PubMed Central. Genome of Alaskapox Virus, A Novel Orthopoxvirus Isolated from Alaska The virus has since been found in red-backed voles and shrews, and domestic cats are implicated as intermediaries that bring it to people.13The Lancet Microbe. Borealpox (Alaskapox) virus: will there be more emerging zoonotic orthopoxviruses? In 2024, one immunocompromised patient in Alaska died from the infection, drawing attention to the potential for previously unknown poxviruses to cause serious illness in vulnerable people.
How Poxviruses Spread
Transmission routes vary by species, but the most common paths are direct contact with skin lesions or respiratory droplets. For smallpox, the relative importance of large droplets versus fine aerosols that linger in the air was debated for decades and never fully resolved.14PubMed Central. What was the primary mode of smallpox transmission? Implications for biodefense Evidence from hospital outbreaks and studies of healthcare workers suggested that fine-particle aerosol transmission could occur, particularly in enclosed settings, but face-to-face close contact remained the most reliable way the virus moved from one person to the next.
Mpox spreads primarily through prolonged skin-to-skin contact, which is why the 2022 outbreak was concentrated in sexual networks. The virus can also spread through respiratory secretions during close interaction, through contaminated bedding or clothing, and from mother to child during pregnancy. Molluscum contagiosum relies almost entirely on direct skin contact or shared objects. Orf virus, as noted, enters through breaks in the skin. Avipoxviruses in wild birds spread through biting insects like mosquitoes and through direct contact with infected individuals.
Environmental stability matters for transmission. The tough, single-membrane particle form of orthopoxviruses can survive on surfaces and in dried scab material for extended periods, sometimes weeks to months in favorable conditions. This is part of why fomite transmission remains a concern for mpox and was historically a worry with smallpox.
How Poxviruses Dodge the Immune System
One reason poxviruses have been so successful is their sophisticated toolkit for evading host defenses. Their large genomes encode dozens of proteins specifically dedicated to interfering with the immune response. Some of these proteins act as decoy versions of host immune molecules: they mimic cytokines, chemokines, or complement proteins and bind to the real versions, neutralizing them before they can activate an immune response.15PubMed Central. Poxvirus immunomodulatory strategies: current perspectives Others block apoptosis, the self-destruct program that infected cells use to limit viral spread. Still others prevent the cell from signaling to its neighbors that it has been invaded.
These so-called host range genes have an interesting wrinkle. While they clearly help the virus manipulate cellular machinery, a critical review found that the number and variety of these genes does not actually predict how many different host species a virus can infect.16PubMed Central. Poxvirus Host Range Genes and Virus-Host Spectrum: A Critical Review In other words, having more immune-evasion tools does not necessarily mean a broader host range. The name “host range genes” is somewhat misleading and may be due for a rethink, since the real function of many of these genes seems to be optimizing replication within particular cell types rather than unlocking new species.17PubMed. Poxvirus host range genes
Vaccines and Cross-Protection
The smallpox vaccine was the first vaccine in history, and vaccinia virus, the virus used in the vaccine, remains central to poxvirus prevention today. Traditional smallpox vaccines used live, replication-competent vaccinia and worked extremely well, but they carried real risks: progressive vaccinia in immunocompromised people, post-vaccinal encephalitis, and eczema vaccinatum in people with skin conditions. The successor vaccine, Modified Vaccinia Ankara (MVA), was created by passaging vaccinia more than 500 times in chicken cells until it lost the ability to replicate in human or most mammalian cells. That makes it far safer while still provoking a strong immune response.18PubMed Central. Immunogenicity and Safety of Modified Vaccinia Ankara (MVA) Vaccine—A Systematic Review and Meta-Analysis of Randomized Controlled Trials A clinical version of MVA, marketed as JYNNEOS in the United States, was the primary vaccine deployed during the 2022 mpox outbreak.
A practical question for public health is how much protection lingers from childhood smallpox vaccination, which ended in most countries by the mid-1970s. A study testing blood from people vaccinated 40 to 60 years ago found that roughly 90 percent still had detectable antibodies against mpox virus, and about 70 percent had neutralizing antibodies. T-cell responses were less durable, turning up in only about 30 percent of vaccinated individuals.19PubMed Central. Evaluation of Cross-Immunity to the Mpox Virus Due to Historic Smallpox Vaccination This partial cross-immunity may explain why mpox outbreaks have been most explosive in younger populations born after routine vaccination stopped. Waning immunity in older adults and the growing pool of unvaccinated younger people are both considered plausible drivers of mpox resurgence.20PubMed Central. Plausible reasons for the resurgence of Mpox (formerly Monkeypox): an overview
Research on next-generation vaccines is active. A synthetic MVA vaccine showed cross-reactive antibody responses against mpox comparable to those elicited by JYNNEOS, and it protected mice from lung infection after mpox challenge.21Communications Medicine. Synthetic modified vaccinia Ankara vaccines confer cross-reactive and protective immunity against mpox virus Another experimental approach uses protein nanoparticle vaccines, which achieved complete protection against lethal orthopoxvirus challenge in mice and outperformed both single-protein vaccines and traditional live-attenuated vaccines.22PubMed Central. Multivalent mpox protein nanoparticle vaccines confer cross-protection against orthopoxvirus infection If these platforms succeed in clinical trials, they could offer broader orthopoxvirus protection without needing a live virus at all.
Diagnostics and Telling Poxviruses Apart
Because many poxviruses produce similar-looking skin lesions, laboratory testing is essential for confirming which virus is responsible. PCR-based methods have been the workhorse for decades. One early approach used primers targeting the hemagglutinin gene, which is specific to orthopoxviruses, to amplify DNA from a sample and then cut it with restriction enzymes to produce fragment patterns unique to each species. This method could distinguish variola from vaccinia, cowpox, mpox, and several others, and even tell apart variola major from the milder variola minor.23PubMed Central. PCR strategy for identification and differentiation of small pox and other orthopoxviruses Multiplex PCR assays that combine genus-specific and species-specific primer pairs in a single reaction have further streamlined the process.24PubMed Central. Multiplex PCR detection and species differentiation of orthopoxviruses pathogenic to humans
Serology, the detection of antibodies in blood, is useful for surveillance but has traditionally struggled with a key limitation: orthopoxviruses are so closely related that antibodies from one species cross-react with proteins from another. It has been hard to tell whether someone’s antibodies came from a past mpox infection, a smallpox vaccination, or both. Newer multiplex antibody assays are solving this by measuring the ratio of responses to specific ortholog pairs. One validated assay achieved sensitivity and specificity both above 95 percent for distinguishing mpox infection from vaccinia vaccination.25PubMed Central. Differentiating mpox infection and vaccination using a validated multiplex orthopoxvirus IgG serology assay That capability matters for epidemiological studies trying to understand who has natural immunity from infection versus vaccine-derived immunity.
Poxviruses as Cancer Therapy Tools
The same features that make poxviruses formidable pathogens, large genomes, cytoplasmic replication, and robust immune-stimulating ability, also make them attractive platforms for cancer therapy. Oncolytic vaccinia viruses are genetically engineered versions of vaccinia that selectively replicate inside tumor cells and kill them while leaving normal tissue largely unharmed. Beyond direct tumor destruction, these viruses can remodel the tumor’s local environment and stimulate long-lasting anti-tumor immune responses.26PubMed Central. Oncolytic vaccinia virus and cancer immunotherapy
Vaccinia’s large genome is an advantage here because it can accommodate big gene insertions, allowing researchers to arm the virus with genes encoding immune-boosting molecules, tumor-targeting proteins, or other therapeutic payloads.27Cancer Biology & Medicine. Preclinical and clinical trials of oncolytic vaccinia virus in cancer immunotherapy: a comprehensive review Multiple engineered oncolytic vaccinia candidates have entered clinical trials, including for liver cancer, and the safety profile of vaccinia in humans is well-characterized from decades of smallpox vaccination.28PubMed. Oncolytic vaccinia virus for the treatment of cancer The field is still maturing, but the idea of repurposing a virus that once killed millions into a weapon against cancer has a certain ironic elegance.
Poxviruses in Wildlife
Poxvirus disease is not just a human concern. Avipoxviruses circulate widely among wild bird populations and can cause two forms of disease: a cutaneous form producing wart-like growths on featherless skin, and a diphtheritic form producing lesions in the mouth and respiratory tract. A molecular survey of wild birds in Central Italy found avipoxvirus DNA in more than 40 percent of sampled birds, even though none of the positive birds showed visible lesions at the time of testing.29PubMed Central. Molecular Detection of Avipoxvirus in Wild Birds in Central Italy That high prevalence of subclinical infection suggests these viruses may be far more common in bird populations than external signs would indicate, with mosquitoes and other biting arthropods acting as key vectors.
In island ecosystems, avipox outbreaks can be devastating to native bird species that evolved without prior exposure. Hawaiian honeycreepers, for instance, have suffered significant population declines attributed in part to introduced avipoxviruses. On the veterinary side, sheeppox and goatpox are among the most economically important animal diseases in parts of Africa and Asia, causing significant losses in livestock herds through mortality, reduced productivity, and trade restrictions. These viruses are distinct from orf and cause a systemic illness rather than localized lesions.
The ecological breadth of the poxvirus family, spanning insects to marine mammals to birds to primates, is remarkable and underscores just how ancient and adaptable this group of viruses is. New species continue to be discovered as surveillance improves, raising the persistent question of which unknown poxviruses may next find their way into human populations.