Animal Hosts

Bats are the natural reservoir of some of the deadliest viruses known: Marburg, Nipah, Hendra, rabies, and the ancestor of the 2003 SARS virus. Ebola is strongly suspected to come from bats too. Their immune systems tolerate viruses without the damaging inflammation that sickens other mammals.

Fun fact: Bats are the only mammals that truly fly; the fever-like heat of flight may help them keep viruses in check.

In 2024, H5N1 bird flu was found spreading among US dairy cattle for the first time, mostly in udders and raw milk, and infected some farm workers. Cattle also suffered rinderpest, a viral plague that was declared eradicated in 2011, only the second disease ever wiped out after smallpox.

Fun fact: The word 'vaccine' comes from vacca, Latin for cow: the first vaccine used cowpox to protect against smallpox.

Pigs are the classic 'mixing vessel' for influenza: their airways carry both bird-type and human-type receptors, so two strains can infect one pig and swap gene segments. The 2009 H1N1 'swine flu' pandemic strain was assembled this way from pig, bird and human flu genes.

Fun fact: Nipah virus first broke out in 1998–99 among Malaysian pig farmers, passed from bats to pigs to people.

Poultry are hit hard by highly pathogenic avian influenza. H5N1 first jumped from chickens to people in Hong Kong in 1997, killing 6 of 18 infected; the city culled its entire poultry population, about 1.5 million birds, to stop it.

Fun fact: Most flu vaccines are still grown inside fertilised chicken eggs.

Dogs are the main source of human rabies worldwide, causing about 99% of cases. Canine influenza H3N8 jumped from horses to racing greyhounds around 2004, and H3N2 jumped from birds to dogs in Asia.

Fun fact: Rabies is nearly 100% fatal once symptoms start, but fully preventable with vaccination.

Wild waterfowl like ducks are the ancient reservoir of influenza A: nearly every subtype (H1–H16, N1–N9) circulates in them, mostly in the gut without making them ill, and spreads through lake and pond water.

Fun fact: Flu virus can stay infectious in cold lake water for months.

Studies in the US found that white-tailed deer can catch some human viruses and pass them between themselves, keeping old strains circulating after they vanish in people.

Fun fact: Deer also carry chronic wasting disease, caused by a prion: an infectious protein, not a virus.

Gulls carry their own flu subtypes (H13 and H16) and helped spread H5N1 along coastlines, where it has killed huge numbers of seabirds and marine mammals since 2021.

Fun fact: Gulls travel between harbours, landfills and farms, making them efficient long-distance virus couriers.

HIV-1 began as SIVcpz, a chimpanzee virus that itself formed when SIVs from two monkey species recombined inside chimps. It crossed into humans around 1920 in southeastern Cameroon, likely through bushmeat hunting, then spread along rivers and railways to Kinshasa.

Fun fact: HIV-2, a milder virus, came separately from sooty mangabey monkeys in West Africa.

Despite their reputation, pigeons are poor hosts for bird flu and rarely spread it. They do carry pigeon paramyxovirus, a relative of the virus behind Newcastle disease in poultry.

Fun fact: Pigeons were domesticated over 5,000 years ago and once carried messages across wars.

H5N1 bird flu reached the sub-Antarctic islands and the Antarctic Peninsula in 2023–24, alarming scientists who watch penguin colonies. Penguins also suffer avian pox and, in warmer places, avian malaria.

Fun fact: Emperor penguins huddle by the thousand to survive −40 °C winters: perfect for keeping warm, and for passing on a virus.

In 1988 and again in 2002 phocine distemper virus killed tens of thousands of harbour seals in the North Sea, and in 2023 H5N1 bird flu killed thousands of elephant seals and sea lions in South America.

Fun fact: Like pigs, seals carry both bird-type and mammal-type flu receptors, so flu strains can mix inside them.

Dromedary camels are the reservoir of MERS coronavirus. Since 2012 it has passed from camels to people, mostly in the Arabian Peninsula, through close contact and raw camel milk, and killed about a third of known cases.

Fun fact: Most camels in the region carry MERS antibodies, so the virus has circulated in them for decades.

In 1994–95 thousands of kangaroos across southern Australia went blind. The cause was traced to Wallal and Warrego viruses, spread by biting midges.

Fun fact: Kangaroos share their country with flying foxes, the bats that carry Hendra virus.

Macaques carry herpes B virus, harmless to them but often fatal to people bitten or scratched without treatment. Rhesus macaques are also the main animal model for HIV research, using its monkey cousin SIV.

Fun fact: Long-tailed macaques live wild in Singapore's nature reserves and Mumbai's hills.

In the Amazon, yellow fever circulates between forest mosquitoes and monkeys. When monkeys start dying of it, health workers know human cases may follow.

Fun fact: Capuchins are among the cleverest monkeys, using stones as tools to crack nuts.

Llamas and alpacas make tiny 'nanobodies', antibody fragments scientists use to design treatments against viruses including HIV.

Fun fact: Llamas are camelids, cousins of the camels that carry MERS.

Lemurs live only on Madagascar. Like other primates they can catch human respiratory viruses, so researchers and keepers wear masks around them.

Fun fact: Ring-tailed lemurs hold 'stink fights', wafting scent from their tails at rivals.

In 2016 a heatwave thawed Siberian permafrost and released anthrax spores from a reindeer carcass buried decades earlier. About 2,300 reindeer died, and a boy was killed.

Fun fact: Melting permafrost may wake other old microbes too; scientists have revived giant viruses frozen for 30,000 years.

Parasitic worms compete with their host for nutrients and can dampen immune responses.

Found in: Fruit Bat, Dairy Cow, Pig, Chicken, Dog, Mallard Duck, White-Tailed Deer, Herring Gull, Chimpanzee, Pigeon, Penguin, Seal, Dromedary Camel, Kangaroo, Macaque, Capuchin Monkey, Llama, Ring-Tailed Lemur, Reindeer.