The majority of human infectious diseases – whether caused by bacteria, viruses or other parasites - could be described as zoonotic, having jumped to humans from animals at some point in history, says Stuart Blackman.
Over evolutionary timescales, crossing the species barrier is the norm rather than the exception. Relative to our own life-spans, though, it’s a rare event. Most pathogens are exquisitely adapted to the particular habitat found within a single host and aren’t capable of venturing further afield. But rich pickings await any parasite that does manage to make the leap to another species. It will find itself within a host that has never had to evolve defences against it, so, for a while at least, it might as well be an all-you-can-eat buffet.
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This is one reason why diseases tend to be at their most deadly soon after they cross a species barrier. As the new host develops defences, virulence declines. But it also pays parasites to evolve mechanisms that reduce their own impact, because a successful pathogen is one that keeps its host alive.
Here are ten deadly zoonotic diseases you really don’t want to catch.
World's deadliest zoonotic diseases
Plague
Few diseases can be said to have been as influential on the course of human history as the plague. An epidemic in the 14th century, known as the Black Death, reduced Europe’s human population of by around 50 per cent. Some historians credit the event with disrupting the feudalism system that was prevalent in Europe at the time, which in turn set the scene for the cultural revolutions of the Renaissance.
The bacterium Yersinia pestis usually lurks within the bodies of a variety of rodent species, but every so often, it spills out from its natural habitat - transmitted by the bites of fleas, which are themselves spread by rats - to infect humans. It is then passed between people via body fluids.
The disease comes in several forms. The most deadly is pneumonic plague, which infects the lungs and is 100 per cent fatal if untreated. Bubonic plague causes pustulous swellings in the groin, neck and armpits and kills around 50 per cent of victims. Septicaemic plague infects the blood and results in the blackening of tissues killed by the infection.
Frequent outbreaks still occur in Madagascar, Democratic Republic of the Congo, and Peru, and an average of seven human cases are reported in the USA each year. A fatal case of pneumonic plague was reported in Arizona in July 2025 having been contracted from an infected animal carcass. The disease can now be treated effectively with antibiotics.
Measles
A measles-like infection has been documented since the 7th Century. The virus that causes it is most closely related to one responsible for a devastating disease of cattle named rinderpest and it is thought that the two diseases were one and the same until they diverged around the 11th and 12th centuries. Since then measles has been an exclusively human disease.
Measles spreads easily in the air via coughs and sneezes and causes a high fever and a dense rash that starts on the face and spreads downwards. The mortality rate is not high (0.2 per cent in the USA), but its infectiousness means that historically it has been contracted by a high proportion of the population, leading to large numbers of fatalities in absolute terms.
The measles vaccine has, according to the World Health Organisation, prevented more than 60 million deaths so far this century. Even so, the disease killed 107,500 people globally in 2023, mostly unvaccinated children under the age of five. The disruption of vaccination programmes during the COVID-19 pandemic has led to an increased risk of measles in 68 countries.
Glanders
Also known as farcy, glanders is a highly infectious bacterial disease of horses, donkeys, and mules. It starts with a fever, mucous discharge and swollen lymph glands in the neck and groin, and progresses to pneumonia, pustules, abscesses and, in the majority of cases, death by septicaemia.
Human cases are rare, and are largely confined to people who work closely with horses. Untreated cases have a 95 per cent fatality rate, although antibiotics can reduce that to 50 per cent. Death takes 7-10 days from the onset of symptoms and some survivors experience chronic illness that can last for years.
During World War I, the Germans infected horses bound for allied nations with the glanders bacterium.
The disease has not been seen in Great Britain since 1928, and the last case in the USA was in 2000, when a researcher was accidentally exposed in a lab, but it still occurs in Africa, Asia, and Central and South America.
AIDS
Infectious diseases are most likely to cross the species barrier when the hosts are closely related. Little surprise that many zoonoses pass to humans from other mammals. Our closest relatives of all, the monkeys and apes, are sources of monkeypox (or MPox), ebola, zika virus, among others.
The human immunodeficiency virus (HIV), which is transmitted sexually and causes AIDS by attacking our immune systems, is descended from a similar virus that is thought to have jumped to humans from chimpanzees in Cameroon early in the 20th Century.
Since HIV/AIDS was first detected in the early 1980s, it has killed at least 40 million people worldwide. AIDS remains incurable, but the development of drugs that suppress the virus mean that it now tends to be regarded as manageable chronic disease rather than an acute and terminal one.
Lyme disease
It’s not the deadliest of zoonoses out there, but Lyme disease is one that can elicit a shudder from many a hardy outdoor enthusiast. It is caused by a bacterium spread by blood-feeding ticks of the genus Ixodes, which lurk in the vegetation ready to hop onto passing mammals. They occur throughout much of the temperate northern hemisphere, especially where deer and sheep are plentiful.
While tick bites can be common, the risk of disease is low. For a start, only 10-30 per cent of ticks carry the bacterium. And, just one per cent of those will transmit the disease in a single bite as long as the tick is removed within 24h. In the UK (population 68 million), there are only 2000-3000 cases of Lyme disease each year, and most years, nobody dies of it. In which case, the chances of death from a single tick bite isn’t far short of one in a million. Feeling lucky?
The only sure way to avoid it completely is to not get bitten. That’s easier said than done, unless you’re happy to stay indoors for the rest of your life. Tucking your trousers into your socks might help, as will insect-repellent. A full body inspection is also recommended on return to civilisation, which is best achieved with the aid of a hand-held mirror or a (very) close friend.
Rabies
Among the most infamous of all zoonotic diseases, rabies was a source of utter terror for many a child growing up in the UK in the 1970s, largely because of the grim tv adverts aired to discourage travellers from smuggling their pets across the English Channel.
Scenes of enormous needles being pushed into the stomachs of children who had been bitten by a stray dog while on holiday were arguably as terrifying as the idea of the disease itself, which causes paranoia, mania, hydrophobia and delirium, as the virus attacks the central nervous system, and ends in an inevitable death.
These days, the fear has subsided significantly. Effective vaccines have been developed, and the UK has remained largely rabies-free. In 2002, a conservationist in Scotland died after being bitten by a Daubenton's bat that was carrying a closely related virus. Prior to that, the last person to die of rabies contracted in the UK was in 1922. However, the disease still claims about 65,000 human lives a year globally, most of them in Africa and Asia.
COVID-19
The precise origin of the SARS-CoV-2 virus that brought much of the world to a standstill and has killed millions of people from 2020 onwards remain mysterious and contentious. But it can be described as zoonotic whether its source was a Chinese wet market or a leak from a research lab.
SARS-CoV-2 bears striking similarities to a group of coronaviruses found in bats, one of which may have infected humans directly or via an intermediate species.
There is also evidence that humans can pass on the infection to animals such as cats and ferrets, a phenomenon known as ‘reverse zoonosis’.
Anthrax
Primarily a disease of grazing animals such as cattle, goats and horses, anthrax is capable of infecting pretty much any mammal, given the chance, including humans.
The bacterium responsible, Bacillus anthracis, disperses as spores, which are either inhaled or enter the body via wounds. Once inside, the bacteria release powerful toxins that destroy host tissues. Infections of the skin result in gruesome black ulcers that can be treated with antibiotics. Lung infections are lethal in 95 per cent of untreated cases, but survival can be increased somewhat if treatment is administered early.
During World War II, British government scientists experimented with anthrax bombs designed to distribute spores over enemy territory. These tests were conducted on Gruinard Island on the west coast of Scotland, which was decontaminated in the 1980s by removing the topsoil and spraying the ground with formaldehyde.
In 2001, five people died after envelopes containing anthrax spores were posted to journalists and senators in the USA.
Toxoplasmosis
Toxoplasma gondii is a tiny single-celled parasite that must pass through two hosts to complete its life-cycle. Having started life in the digestive tract of a member of the cat family, it must then seek out an intermediate host - almost any warm-blooded animal will do – and lodge itself away in its tissues, ready to be eaten by another cat.
Humans can become infected by consuming under-cooked meat or via exposure to cat faeces. The microbes can also be passed from mother to child during pregnancy.
Most human infections are asymptomatic, although sometimes it can lead to a serious flu-like illness. In the USA, deaths number a few hundred per year, making toxoplasmosis the second most common cause of death from food (after Salmonella poisoning).
However, there’s evidence that even asymptomatic T. gondii infections are dangerous. Infected individuals are significantly more likely to die of any cause over a given timescale compared to non-infected people. Infection also seems to be associated with higher rates of schizophrenia, bipolar disorder, risky behaviours and suicide.
Given that an estimated 30 to 50 per cent the world’s population are infected with T. gondii, that could all add up to a lot of deaths. However, research in this area is still at an early stage, and a causal link has not been established. It might yet turn out that people with existing health problems are more likely to contract T. gondii rather than vice versa.
Avian Influenza
A virulent form of avian influenza - better known as ‘bird flu’ - is currently wreaking havoc upon the world’s feathered inhabitants.
The H5N1 virus was first detected in 1959, and since 1997, hundreds of millions of farmed birds have been slaughtered in huge effort to contain its spread. Since 2021, a particularly deadly variant has become dominant and has spread to all continents except Australia. In 2022, it reduced the world’s largest breeding colony of gannets, on Bass Rock in Scotland’s Firth of Forth, by 25 per cent.
It has also started infecting mammals, including bears, cattle, seals, cats, and otters. To date, fewer than 1,000 cases have been documented in humans, most of them involving people who work closely with farmed birds, but the mortality rate may be as high as 50 per cent. In a small handful of cases, the virus has been transmitted between people, though not at a rate that would sustain a human pandemic.
H5N1 is not up there with the deadliest of zoonotic diseases – yet. But, needless to say, scientists are monitoring developments very closely.
Top image: Getty
