A bat or a rat or a chicken has a virus. And somehow a person catches it — maybe by contact with the animal’s blood or feces, maybe just by breathing in.

That’s a spillover.

We’ve been exploring the topic of spillover viruses this year — when animal pathogens jump into people and can lead to outbreaks, even pandemics.

In a series of radio and digital posts, we’ve covered a variety of viruses, from Marburg to Nipah to a mysterious new coronavirus found in Malaysia and Florida. We’ve interviewed disease detectives, and we’ve looked at how to stop the next pandemic.

We also asked you to send us your questions on hidden viruses. Here are some of the queries, answered by the correspondents who contributed to our series.

Get ready for a silly question: I love my pups very much – and I think they love me too because I get lots of kisses. Is that bad from a spillover virus perspective – for me or my dogs? Should I train my pups to be less … kissy?

We actually thought this question was relevant to the millions of people who enjoy a furry companion, so correspondent Michaeleen Doucleff wrote a separate story about it.

They may indeed be giving you some germs with their love! Find out what that might mean.

I just read your story about viruses jumping from animals into people frequently and I have a question: Do all viruses that jump make us sick? Is there a virus that confers an advantage to its host? — Kaveh Shoorideh

No, not all viruses that make this jump cause us to become ill. In fact, the vast majority of spillover viruses don’t make people sick.

Even if a virus does make a human sick it won’t necessarily spread. A virus that’s too deadly will kill its hosts and peter out quickly, and a virus that’s not sufficiently transmissible simply won’t spread.

The concern arises when a virus that’s spilled over into humans causes some people to become very sick and infect others. Severe illness combined with high transmissibility makes for a virus that can spread and cause a lot of damage in its wake.

It’s unclear whether any spillover viruses are advantageous, but scientists do know that viruses more generally can be helpful to humans. Just take the fact that viruses have shaped our evolution. One study estimated that viruses have driven the adaptation of some 30 percent of mammalian proteins! They’ve been at least partly responsible for humanity’s strong immune response to pathogens. Some viruses in our gut may even protect us from autoimmune diseases, such as Crohn’s. And they’re used in laboratories worldwide as tools for research (to help map circuits in the brain, for example). — Ari Daniel

I’m assuming spillovers having been occurring since the beginning of time. Is there an uptick correlated to climate change? — Debbie Stavish

Climate change is indeed of great concern to scientists when it comes to increasing the opportunities for spillovers of viruses into humans.

As average temperatures and humidity rise in a given region, virus-carrying mosquito species and animals that previously could not thrive there can now move in. Climate change has also increased the incidence of extreme weather events – massive rains and flooding are just one example – that have forced millions of people out of their homes and into camps where a virus can more easily spread.

These new types of climate-driven situations obviously expand the chances for a previously unknown virus to make the jump from an animal into humans. But just as worryingly, they may amplify the incidence of existing spillovers that have already been happening at low levels for decades – threatening to turn minor nuisances into major disease outbreaks. An example is the mosquito-driven Zika epidemic that spread through the Americas from 2015 through 2016, causing thousands of children to be born with birth defects, including microcephaly, a medical condition involving a smaller than normal head and brain damage. Climate change could lead to longer warmer periods in which the mosquito that transmits Zika could thrive.

South African-based Tulio de Oliveira is a researcher who helped launch an international effort to combat precisely this sort of climate change-amplified spillover possibility. The idea is for ecologists and data experts to collaborate with disease experts to hone in on locations where climate change seems particularly likely to amplify spillover events. Researchers will then make a point of taking blood samples and otherwise monitoring people in those areas to identify new pathogens circulating there and get the jump on developing diagnostics, therapeutics and vaccines against any new disease they find. That way, says de Oliveira, “if it starts to spread around the world, we’ll know how to respond.” — Nurith Aizenman

Why are bats so often linked to spillover events and other pandemic outbreaks? Do they carry more viruses than other animals, and if so, why? Thanks for doing this series! — Jodi McClory

There’s some fascinating work looking at bat biology and how the immune system of these animals may allow them to carry a large number of viruses without getting sick. It has to do in part with the bats’ ability to suppress inflammation. They’re the only mammal capable of flight, which requires a high metabolism to keep up. But that creates more stress on their bodies, which causes DNA damage and inflammation. Bats have evolved ways of minimizing this damage and repairing it when it occurs. By boosting their immune response, this ability to self-repair could keep viral infections circulating in their bodies without leading to illness.

In addition, there’s still some debate as to whether bats host more viruses than other mammals, but Cara Brook, a disease ecologist at the University of Chicago, says, “it is generally accepted that they host the most virulent viruses known to people.” — Ari Daniel

What components of a virus most frequently change during a spillover event? Spike proteins? Proteins involved in replication? And how do these changes contribute to allowing for successful spillover? Are certain types of viruses more likely to spillover than others and, if so, what types? What are the differences in symptoms of infection and transmissibility between the original animal viral host and the spillover animal host? — Heather Thompson

I’ll take this in parts: One research study found that, “the majority (94 percent) of zoonotic viruses [spillover viruses] described to date are RNA viruses.” The authors point out that this is 28 times higher than the proportion of RNA viruses among all viruses in vertebrates, “indicating that RNA viruses are far more likely to be zoonotic than DNA viruses” because they tend to transmit and evolve more rapidly.

A virus can act differently in different species. For example, the species in which a virus primarily lives and reproduces, called the reservoir species, usually doesn’t show signs of illness.

As for which components of a pathogen are involved in a spillover, a virus can benefit if it targets something in one species that’s shared across other species. For instance, SARS-CoV-2 enters the cell after attaching to the ACE2 receptor in our bodies (primarily in the lungs). A lot of other vertebrates have a similar receptor, which means that the coronavirus has found its way into many other animals, including deer, mink and hamsters. — Ari Daniel

I seem to recall a spillover virus that was killing horses in Australia. At some point it killed a horse trainer… [but] it never jumped global. — James Day

How likely is the virus Hendra to be a pandemic one day? — Rachel Barnes

The horse-killing virus was Hendra — and it’s a dangerous disease. Bats are the natural reservoir. It doesn’t make them ill but they shed the virus through their waste, and when horses come into close contact with bat urine or feces, they can catch Hendra. The horses that have been infected developed a frothy, nasal discharge and odd behaviors such as drinking water incessantly or throwing themselves against the walls of their stable. On a few occasions, people from in and around Brisbane and Queensland, Australia interacting with these horses then got sick from Hendra in a second spillover, including a trainer. Of the seven people who’ve had Hendra, four have died.

Bats are more likely to shed Hendra virus when they’re not eating enough, a scenario that’s become more common due to climate change and habitat loss. But periods of winter flowering have provided bats with enough food to reduce the likelihood of spillover dramatically. Preventing animals other than bats from getting sick lowers the chances that Hendra could become a global pandemic in humans one day. — Ari Daniel

Can frequent spillovers provide population-wide low-level immunity to viruses such as COVID-19? I ask because COVID-19 didn’t get a grip in Cambodia until late February 2021 when tourists brought in the alpha variant. The original variant came into Cambodia a few times but the outbreaks died out. I’ve been thinking that one reason for the low transmission rate early on was that people had already been exposed to similar viruses via bats. — Susan Smith

There’s some evidence for this idea that “repeated low-dose exposure can increase host immunity to infection,” according to one team of researchers. This may be the reason, for instance, why those who handle poultry have been found to be less susceptible to avian influenza. And it may explain why some slaughterhouse workers in Nigeria had increased immunity to a different kind of coronavirus (called MERS-CoV) found circulating in the dromedary camels that were being slaughtered. The workers displayed no symptoms, leading one article to summarize, “The immunity that these individuals had acquired from, presumably, camel-adapted variants might be a further barrier to spillover of more human-adapted strains.”

But there are counter-examples from the world of bacteria. Tannery workers exposed to Bacillus anthracis, the bacterial species that cause anthrax, don’t appear to have heightened immunity against the pathogen. So a population’s ability to acquire low-level immunity to a pathogen is complex and likely depends on different factors. — Ari Daniel

Have we learned enough from this pandemic to prevent the rapid spread of a new virus? — Matt Giaquinto

The COVID-19 pandemic has certainly raised global awareness of the power of a new and deadly virus. In some ways, it comes down to what was learned about public health messaging. That is, as researchers developed a better understanding of how SARS-CoV-2 was transmitted, health officials honed their recommendations to advise the public on how best to contain its spread. While the next pathogen with pandemic potential may be transmitted differently than COVID (a gastrointestinal virus, for instance, behaves quite differently from a respiratory virus), saving lives will still come down to providing clear, accurate and timely information and developing partnerships with trusted community leaders — even if some distrust the messaging.

On the science side, researchers developed vaccines in record speed to combat SARS-CoV-2. The most widely used vaccines rely on new mRNA technology. These advancements will undoubtedly be of great help when the next unknown pathogen surfaces since mRNA vaccines can be developed quickly and flexibly. For an explanation of how mRNA vaccines work, here’s an excellent Short Wave episode.

And taking the 10,000-foot view, scientists are also getting better at knowing how to search for viruses of concern that have spilled over from animals. A more efficient hunt for problematic pathogens may help us stay a step or two ahead of a virus to shut it down before it becomes a full-blown pandemic. — Ari Daniel

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