When Viruses Jump Hosts

Viruses Are Everywhere

To infect a cell, a virus needs to be able to lock on to surface receptors. Most viruses in the world can’t lock on to human cells—so they can’t get in.

Viruses are a natural part of all ecosystems. There are viruses that infect all types of living things—plants, fungi, bacteria, animals, and more. There are even viruses that hitch a ride on other viruses!

Fortunately, most of the millions of viruses you bump into, breathe in, and swallow every day can’t hurt you. That’s because their surface proteins don’t fit with your cell receptors. They can’t “lock on” and infect your cells.

That is, most of the time.

To learn how viruses infect cells, visit How Viruses Work.

Viruses that Infect Similar Hosts Are More Likely to Cross Over

Once in a while, viruses that infect animals do cross over to people. When a virus infects a member of a new species, it’s called a spillover infection.

Spillover infection is most likely to happen between species that are closely related. For example, a virus that infects plants or bacteria is not going to infect you. People are just too different from their regular hosts. When viruses spill over to people, it’s usually from another mammal or sometimes birds. That’s because closely related species are more likely to have surface receptors in common that viruses can use to get into cells.

Think about other mammals like cows, cats, and bats. They all have a lot of the same organs and cell types as people. We all breathe air through our noses and into our lungs. Inside our bodies, the air passes by the same cell types, which use similar proteins to do the same jobs. There are differences—your cell surface proteins aren’t exactly like your cat’s. But sometimes they’re alike enough that a virus can lock on and get in.

Some Viruses Have a Broad Host Range

Viruses that infect multiple hosts are more likely to spill over. Related strains of influenza A virus infect wild birds, as well as farm animals, dogs, and more.

Some viruses infect just one host species. They’re “specialists,” and they don’t spill over very often. In contrast, other viruses can infect multiple hosts. These “generalists” are more likely to spill over to new host species. For example, related strains of norovirus can infect people, farm animals, pets, and rodents.

In fact, animals can be reservoirs for some viruses that also infect people. This means that the virus spreads in a group of animals, then spills over to humans when it has a chance. Ebola virus, for example, has made the jump to humans multiple times since the 1920s, most likely from bats.

For a virus, having a broad host range is a good thing. More hosts means more chances to spread. But just because a virus can get into a new host species doesn’t mean it will.

Zika virus is carried by mosquitoes. It can infect several farm animals, rodents, bats, orangutans, and more.

Spillover Requires Contact

To share viruses, species need to come into close contact with one another. And some viruses that could infect a different host don’t, just because they never have the chance. Chances for contact are affected by the population size, density, location, and behavior of both species.

The bad news for us is that we’ve built a world in which it is easier than ever for viruses to jump from animals to people. And there’s some evidence that spillover to humans is happening more often. There are more people on our planet than at any other time in history. Communities of people are pushing into animals' habitats. And we invite them to move into ours—by building mine shafts and growing and storing grains. People even capture wild animals to sell for a profit.

Modern farming practices also invite new viruses into our communities. We raise livestock in huge numbers, often crowded together. Multiple times, viruses have jumped from wild animals to farm animals and then to people. The 2003 SARS outbreak involved a virus that moved from wild bats to farmed civets and then to people. And there have been hundreds of cases of people on poultry farms catching deadly strains of influenza A, the flu virus, carried by wild birds. Fortunately, most “bird flu” cases remain isolated.

Bird flu shows that even when viruses spill over, they can’t always spread. Likewise, even though the rabies virus infects many hosts—including wild animals, pets, and livestock—it rarely spreads from person to person.

Spillover Requires Biological Compatibility

A virus can enter a host cell only if its proteins can attach to host cell receptors. But that’s just one of many steps in a successful infection.

The virus also has to

  1. get its genetic information into the host cell,
  2. get the cell to read its genes and make more viruses, and
  3. get the cell to release the new viruses, which spread to other host cells.

At each step, parts of the virus interact directly with parts of the host. To succeed, a virus needs to have a good fit with the host cell. Only then can a virus cause an infection in a new host.

How a Spillover Grows into an Outbreak

Even though it’s fairly common for viruses from other species to spill over and infect humans, few can then spread from person to person.

To spread in a new host, the virus has to get past some barriers. Below are some important factors:

  • Fit with the new host. Above all, a virus needs to be well-adapted to its host. Most of the time when a virus infects a new host, a series of genetic changes must happen that make it a better fit. To learn more about how these changes happen, visit How Viruses Evolve.
  • How it spreads. Rabies can infect lots of mammals, including people. But it hardly ever spreads between people. That’s mainly because rabies spreads through infected saliva ("spit"), and we usually don’t bite or scratch one another. A virus that spreads through droplets that we breathe out—like coronavirus or influenza—has a much better chance of spreading between people.
  • Structure of the host population. To succeed, a virus needs a host population that’s large and connected enough to maintain its spread. If an outbreak is in a small, remote community, the virus has few hosts to spread to. That’s what happened for many years with Ebola; outbreaks were small and contained. But more recently, like in the 2014 outbreak, the virus reached large cities where it spread through many more people. Unfortunately, growing populations and ease of travel make it easier for viruses to spread—as we also saw with the fast spread of SARS-CoV-2.

Examples of Viruses That Have Spilled Over to People

Disease or VirusAnimal HostWhen It Jumped to HumansNotes
Measles morbillivirusCattleApprox. 1000 - 1200From historical records, it looks like this virus may have jumped multiple times, starting when people began raising cattle. The strain that’s common today probably jumped in the 11th or 12th century.
Dengue & Yellow FeverNonhuman primatesProbably thousands of years agoThese mosquito borne viruses most likely started in Africa. Then human travelers spread them to Asia, Oceania, and the Americas. Yellow fever also spread to Europe.
Influenza AWater birds (e.g., ducks)Multiple times, including 1912, 1957, 1968Water birds (e.g., ducks) are the natural hosts of this diverse group of viruses. New strains have arisen and spread to other animals (wild and domestic) and people.
HIV/AIDSChimpanzees, monkeys1930sThis virus jumped to humans at least 4 times, giving rise to multiple viral strains. Just one of these strains, from Chimpanzees, led to the HIV pandemic.
EbolaProbably fruit batsMany times, including 1976, 1994, 2014Ebola passes to humans from fruit bats. Bats may be a reservoir for the virus, or they may catch it from a different animal.
SARS-CoV-1 novel coronavirusBats, civets2003This virus most likely came from bats. It likely jumped to civets (and possibly other animals) and then to people.
H1N1 "swine" flu strain of influenza APigs, water birds2009This strain started in water birds. It jumped to farmed pigs, then to people.
MERS novel coronavirusBats, camels2012This virus most likely started in bats then spread to camels. Most people get MERS from camels; it doesn’t spread well between people. MERS is known for its high fatality rate, around 35%.
Zika virusNonhuman primatesMany times, including 2015 epidemicThis mosquito-borne virus was first found in African monkeys. Infected people brought it to many other places, including South America. Mosquitoes then spread the virus to wild primates, which are a reservoir.
SARS-CoV-2 coronavirusBats2019This virus most likely started in bats. It likely jumped to another animal host before infecting people.

Predicting and Preventing Spillover Events

Even when viruses become able to spread from person to person, few go on to spread around the world. Yet when they do, the effects can be deadly. Efforts to predict and prevent spillover events are our first line of defense.

Often when viruses spill over, they spread from wild animals to farm animals and then to people. For example, MERS jumped from bats to camels, then to people. H1N1 “swine” flu arose in farmed pigs and then spread to people. It’s important to check farm animals for novel viruses. That way, the animals can be treated, vaccinated, or culled before the virus has a chance to jump to people.

Research into viruses that infect wild animals can tell us a lot. By comparing animal viruses to the ones that infect people, scientists can learn where new viruses have come from in the past. This information can help them find places where future outbreaks might start. Then they can monitor the communities where people have contact with the animals that pose a risk. These efforts can help to both identify and prevent new outbreaks.

Farm animals and pets can also be a target for tools that prevent their spread in people. There is no MERS vaccine yet for people, but one is being made for camels. This vaccine could break the link in the spread to people.

Detecting Outbreaks

Once a virus spills over to humans, the goal is to detect it before it can spread. Global task forces, like the World Health Organization (WHO), help build and coordinate worldwide surveillance efforts. Surveillance efforts watch closely for signs of outbreaks. Some viruses, like Ebola, cause such severe illness that they are fairly easy to detect. Infected patients often end up in hospitals, where doctors can make a diagnosis. A diagnosis is even easier if a virus causes distinct symptoms.

In contrast, viruses that cause mild to moderate respiratory symptoms, like influenza A and coronaviruses, can be tricky to notice. Unfortunately, these viruses are also among the most likely to spill over from animals to humans. Novel strains of influenza appear every few decades. And since the turn of the century, we’ve had multiple outbreaks of novel coronaviruses, including SARS in 2003, MERS in 2012, and COVID-19 in 2019.

In the past, it was much harder for scientists to find the sources of new outbreaks. Today, they can detect and identify a virus by its genetic information. Tools like PCR and DNA sequencing make detecting outbreaks faster than ever.

Once experts detect a new outbreak, efforts shift to slowing its spread. Groups like WHO also coordinate efforts to develop tools like diagnostic tests and vaccines.


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