What Diseases Do Monkeys Carry That Infect Humans?

Monkeys and other non-human primates carry a remarkably wide range of pathogens that can infect people, from retroviruses and herpesviruses to malaria parasites and intestinal worms. The reason is straightforward: humans and non-human primates share between 75% and 98.5% of their DNA, which means our cells, immune responses, and tissue structures are similar enough that many of the same microbes can thrive in both.¹ That genetic closeness makes primate-to-human disease spillover more likely than transmission from more distantly related animals, and it has shaped some of the deadliest epidemics in modern history.

Why Monkeys and Apes Are Uniquely Risky

Not all animal-to-human disease jumps are equally likely. A virus adapted to a frog faces enormous biological barriers before it could replicate inside a human cell, but a virus circulating in a macaque already knows how to use molecular machinery that closely resembles ours. Non-human primates and humans have highly similar tissue structures, immune signaling, and metabolic pathways, so a pathogen that has evolved to exploit one primate species often needs only minor adaptations to infect another.¹ This is why non-human primates are a central focus of zoonotic disease research: they represent a disproportionate source of emerging infections that threaten people.

That said, molecular barriers still exist. Cells carry antiviral proteins that can block foreign viruses at specific steps of replication. In the natural host, the virus has evolved a countermeasure, but in a new host species those defenses may hold. One well-studied example involves a protein called APOBEC3G, which can scramble the genetic code of retroviruses. Viruses jumping between primate species have had to evolve workarounds to defeat this defense in their new hosts, and research on how SIV adapted to overcome APOBEC3G in macaques has confirmed that these molecular barriers are real bottlenecks to cross-species transmission.² Sometimes the virus clears the barrier; sometimes it does not. When it does, the consequences can be enormous.

HIV and Other Retroviruses

The most consequential monkey-to-human disease jump in recorded history is HIV. Both HIV-1 and HIV-2 originated from simian immunodeficiency viruses (SIVs) that naturally infect African primates. More than 40 species of African monkeys carry their own species-specific forms of SIV, and in many of those hosts the infection appears to cause little or no disease.³ Chimpanzees acquired SIV from monkeys and passed a recombinant form onward. One particular transmission event, involving SIV from chimpanzees in southeastern Cameroon, gave rise to HIV-1 group M, the strain responsible for the global AIDS pandemic.⁴ At least four independent cross-species transmissions of SIV to humans have been documented, with one or two of those possibly passing through gorillas along the way.³

HIV is not the only retrovirus to have made the jump. Simian T-lymphotropic virus 1 (STLV-1) enters human populations through contact with non-human primate bushmeat, and research in tropical West and Central Africa has documented high levels of exposure among people who hunt and butcher primates.⁵ A related pathogen, simian foamy virus (SFV), also crosses into people who have direct physical contact with non-human primates. In one study of people bitten or scratched by primates while hunting in Cameroon, roughly a third of those who had encountered an ape tested positive for SFV, compared with about 5% of those whose contact was limited to smaller monkeys.⁶ Broader surveys in Central Africa have found SFV infection rates approaching 19% among people with direct primate contact, versus a fraction of a percent in the general population.⁷ SFV does not seem to cause disease in the people it infects, but its ability to cross the species barrier so readily is a reminder that new primate retroviruses could emerge at any time.

Herpes B Virus

Macaque monkeys carry a herpesvirus called B virus (also known as herpes B or Cercopithecine herpesvirus 1) that is the mirror image of the cold sores most people are familiar with. In macaques, it produces mild or invisible infections and is extremely widespread, found in roughly 80% to 90% of adult animals.⁸ In humans, though, the virus behaves very differently. Untreated B virus infections can progress to severe brain inflammation, and the mortality rate without timely treatment has been estimated at 70% to 80%.⁹ Survivors often have lasting neurological damage.

Human infections are rare overall because sustained close contact with macaques is needed, but B virus is a constant concern for laboratory workers, zoo staff, and anyone else who handles macaques. The risk is also elevated at Asian monkey temples, where thousands of tourists interact with free-ranging macaques every year. The dynamics at these sites differ from controlled laboratory settings, and the frequency of bites, scratches, and mucosal splashes may carry a higher risk of primate-to-human viral transmission than encounters in more regulated environments.¹⁰

Monkey Malaria

Malaria is usually thought of as a disease carried by four human-adapted species of Plasmodium, but a fifth species, Plasmodium knowlesi, is a malaria parasite that naturally infects macaques in Southeast Asia. Long-tailed macaques, pig-tailed macaques, and a third macaque species serve as its natural hosts, and the parasite reaches people through the bite of forest-dwelling Anopheles mosquitoes that feed on both monkeys and humans.¹¹ P. knowlesi is now recognized as the fifth species causing human malaria.¹²

Naturally acquired human cases were thought to be extremely rare until a large cluster was identified in Sarawak, Malaysian Borneo, in 2004. Since then, human infections have been described throughout Southeast Asia, with Malaysia reporting the largest number.¹³ The rising case count is partly explained by better diagnostic tools that can now distinguish P. knowlesi from the similar-looking P. malariae under a microscope. But environmental change is also driving exposure: deforestation pushes people, mosquitoes, and macaques into closer contact, increasing the opportunities for the parasite to jump between species.¹³ P. knowlesi malaria is primarily a zoonosis rather than a newly emerged disease; humans have likely been getting infected for a long time, but we only recently had the tools to notice.

Intestinal Parasites Shared Between People and Primates

The parasites that pass between monkeys and people are not limited to malaria. Several gut parasites circulate in both directions. Cryptosporidium hominis, a waterborne parasite initially described as infecting only humans, has been found in non-human primates living near human settlements in Ethiopia, likely as a result of contaminated food and water in areas where primates increasingly venture into agricultural fields and homes.¹⁴ Giardia duodenalis, another common intestinal parasite, has been identified in wild macaques carrying genetic forms known to infect people, though the precise zoonotic risk is still debated because single-gene typing can overestimate how similar the monkey and human strains truly are.¹⁵

Parasitic worms add another layer. Oesophagostomum, a nodule worm, infects both wild primates and people in Sub-Saharan Africa. Research in western Uganda has identified multiple cryptic forms of the parasite circulating in people and nearby primate populations, with different parasite lineages varying in their ability to cross between host species.¹⁶ This worm had previously been recognized as a human health concern only in Togo and Ghana, so the Ugandan findings suggest human infections may be more widespread than previously known.

How People Get Exposed

The routes of transmission vary depending on context, but they generally fall into a few categories:

Bushmeat hunting and butchering: Handling primate carcasses exposes people to blood and other bodily fluids, and this is the route through which retroviruses like SIV, STLV-1, and SFV most commonly cross into humans.¹⁷
Bites and scratches: Direct physical contact at monkey temples, in laboratory or zoo settings, or during encounters with wild or semi-wild monkeys in urban fringe areas can transmit blood-borne pathogens and herpes B virus.
Mosquito vectors: Diseases like P. knowlesi malaria and yellow fever do not require any direct contact with monkeys at all. A mosquito bites an infected monkey, then bites a person.¹⁸
Shared environments: Where monkeys and people live in close proximity, fecal contamination of water and food can transmit gut parasites in either direction.
Exotic pet ownership: Keeping primates as pets creates prolonged close contact. Recent outbreaks of zoonotic diseases in humans have been associated with exotic pet keeping, and expanding human populations and increased exotic pet ownership may contribute to rising incidence of these diseases.¹⁹

The risk is not evenly distributed across the globe. Tropical Africa and Southeast Asia are hotspots, both because primate diversity is highest in those regions and because the overlap between human and primate habitats is most intense. Deforestation, agricultural expansion, and urbanization keep pushing that overlap further.

Diseases Blamed on Monkeys That Are Not Really Theirs

Two high-profile diseases illustrate how the monkey label can be misleading. Mpox (formerly monkeypox) was named after the laboratory monkeys in which the virus was first identified in 1958, but monkeys are not thought to be the natural reservoir. Ecological modeling has pointed to arboreal rodents, particularly African squirrels, as the most probable maintenance hosts for the virus.²⁰ Monkeys can get infected and can pass the virus to humans, but they appear to be spillover hosts rather than the animals keeping the virus circulating in nature.

Ebola is a similar story. When human Ebola outbreaks occur, they are frequently preceded by observed die-offs in wild primate populations, which has led to a common assumption that apes and monkeys are the source. But the ebolaviruses are typically highly lethal to non-human primates, just as they are to us, which makes it unlikely that primates serve as long-term reservoirs. Instead, primates appear to be amplification hosts or spillover victims that catch the virus from an as-yet-unconfirmed reservoir, likely bats, and then pass it to humans who encounter sick or dead animals.²¹ The distinction matters: controlling Ebola in the long run requires identifying and managing the true reservoir, not just monitoring primate die-offs after the fact.

Tick-Borne and Mosquito-Borne Viruses With Primate Links

Yellow fever remains one of the most important mosquito-borne diseases with a primate reservoir. The virus cycles between non-human primates and forest-dwelling mosquitoes in tropical Africa and the Americas. When a person enters the forest and is bitten by an infected mosquito, they bring the virus back into urban areas, where it can trigger explosive outbreaks spread by a different mosquito species.¹⁸ Vaccination is highly effective, but the ongoing sylvatic cycle in wild primates means yellow fever cannot be eradicated and will continue to pose a threat wherever vaccination coverage is incomplete.

Less well known is Kyasanur forest disease (KFD), a tick-borne viral hemorrhagic fever found in India. The virus circulates among primates and other wildlife, reaching people via tick bites in rural and forested areas.²² KFD is geographically limited and not well known outside India, but it demonstrates that the overlap between primate habitats and human settlements creates vulnerability even for diseases that do not make global headlines.

When the Traffic Runs in Reverse

The disease exchange between monkeys and humans is not one-directional. People also infect primates, a phenomenon called reverse zoonosis or anthroponosis, and this bidirectional flow complicates conservation and public health alike.

Wild chimpanzees across Sub-Saharan Africa have suffered lethal outbreaks of respiratory viruses traced back to human origins. In one case, simultaneous outbreaks of two distinct human respiratory viruses hit two chimpanzee communities in the same Ugandan forest at the same time: human metapneumovirus and human respirovirus 3, both of which clustered phylogenetically with known human variants.²³ Research has since shown that these “common cold” pathogens circulate among children, infect adults without causing symptoms, and those asymptomatic adults then unknowingly carry the viruses into the forest and infect chimpanzees.²⁴ Because the adult carriers do not feel sick, intervention strategies based on screening for respiratory symptoms in people entering primate habitat are unlikely to work.

Tuberculosis provides another example. In a captive colony of Aotus monkeys (owl monkeys), a TB outbreak was traced to continuing intermittent exposure from a human source rather than monkey-to-monkey spread.²⁵ Surveys of synanthropic macaques (those living alongside humans, in temples or urban edges) have found that the prevalence of Mycobacterium tuberculosis complex DNA in those monkeys tends to be higher in countries where human TB rates are higher, pointing to humans as the infection source rather than the other way around.²⁶ Even a bacterial pathogen like Campylobacter jejuni has been found shared between captive macaques and the humans who care for them, with genetic analysis suggesting the monkeys caught it from their human handlers or from a shared contamination source rather than bringing it in from the wild.²⁷

The practical implication of reverse zoonosis is that protecting monkey and ape populations from human diseases is not just a conservation concern. Primates that acquire human pathogens can potentially evolve and recombine those pathogens in new ways, creating a theoretical risk that a modified version could jump back into the human population. Keeping the traffic low in both directions is the safest approach for everyone.

Deforestation, Urbanization, and Rising Risk

Almost every disease on this list is becoming more relevant, not less. The underlying driver is habitat overlap. When forests are cleared for agriculture or settlement, the monkeys that lived there do not simply vanish. Many primate species adapt to fragmented landscapes and begin raiding crops, visiting trash dumps, or settling near temples and tourist sites. That behavioral flexibility brings them into repeated contact with people, livestock, water supplies, and domestic animals.

P. knowlesi malaria offers the clearest illustration. The rising number of human cases in Malaysia has been linked not only to better diagnostics but to deforestation increasing human exposure to the forest-edge mosquitoes that bridge the gap between macaques and people.¹³ The same dynamic plays out with bushmeat hunting in Central Africa, where expanding human populations push deeper into primate habitat and increase the frequency of blood-borne exposures that allow retroviruses to cross over.¹⁷ Even gut parasites like Cryptosporidium hominis spread more readily when primates regularly forage in human-managed landscapes and contaminate the same water sources people rely on.¹⁴

Zoonoses from non-domestic animals already make up roughly three quarters of today’s emerging infectious diseases.¹⁹ Primates, because of their genetic closeness to us, occupy a uniquely dangerous niche within that broader trend. Each new road cut into tropical forest, each expanding farm that borders monkey habitat, and each exotic primate purchased as a pet slightly widens the doorway through which the next pathogen could walk.

1
PubMed Central. A Review on Zoonotic Pathogens Associated with Non-Human Primates: Understanding the Potential Threats to Humans
2
PubMed Central. APOBEC3G polymorphism as a selective barrier to cross-species transmission and emergence of pathogenic SIV and AIDS in a primate host
3
PubMed Central. The evolution of HIV-1 and the origin of AIDS
4
PubMed Central. Origins of HIV and the AIDS pandemic
5
PubMed Central. Bushmeat Hunting and Zoonotic Transmission of Simian T-Lymphotropic Virus 1 in Tropical West and Central Africa
6
PubMed Central. Simian Foamy Virus Transmission from Apes to Humans, Rural Cameroon
7
PLoS Pathogens. Frequent and Recent Human Acquisition of Simian Foamy Viruses Through Apes’ Bites in Central Africa
8
PubMed Central. B-virus from pet macaque monkeys: an emerging threat in the United States?
9
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10
Emerging Infectious Diseases. Temple Monkeys and Health Implications of Commensalism, Kathmandu, Nepal
11
PubMed. Human Plasmodium knowlesi infections in South-East Asian countries
12
PubMed Central. Human infections and detection of Plasmodium knowlesi
13
PLOS Neglected Tropical Diseases. Plasmodium knowlesi infecting humans in Southeast Asia: What’s next?
14
PubMed Central. Genetic diversity of Cryptosporidium spp. in non-human primates in rural and urban areas of Ethiopia
15
International Journal for Parasitology: Parasites and Wildlife. Occurrence of Giardia, Cryptosporidium, and Entamoeba in wild rhesus macaques (Macaca mulatta) living in urban and semi-rural North-West India
16
PubMed Central. Nodule worm infection in humans and wild primates in Uganda: cryptic species in a newly identified region of human transmission
17
One Health. High potential for zoonotic pathogen emergence through wildlife exposure in East Cameroon
18
PubMed Central. Yellow fever: a reemerging threat
19
PubMed Central. One health: zoonoses in the exotic animal practice
20
PubMed Central. Identifying the Most Probable Mammal Reservoir Hosts for Monkeypox Virus Based on Ecological Niche Comparisons
21
PubMed Central. African Primates: Likely Victims, Not Reservoirs, of Ebolaviruses
22
PubMed Central. Kyasanur Forest Disease: A Comprehensive Review
23
PubMed Central. Simultaneous outbreaks of respiratory disease in wild chimpanzees caused by distinct viruses of human origin
24
Scientific Reports. Common cold viruses circulating in children threaten wild chimpanzees through asymptomatic adult carriers
25
PubMed. Tuberculosis outbreak in a closed Aotus monkey breeding colony: Epidemiology, diagnosis and TB screening using antibody and interferon-gamma release testing
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PubMed Central. From the mouths of monkeys: detection of Mycobacterium tuberculosis complex DNA from buccal swabs of synanthropic macaques
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PubMed Central. Evidence of potential Campylobacter jejuni zooanthroponosis in captive macaque populations