It’s no coincidence that some of the worst viral outbreaks in recent years (SARS, MERS, Ebola, Marburg, and maybe even SARS-CoV-2) emerged in bats. Due to their biological peculiarities and lifestyle, bats are a so-called virus reservoir — and while we don’t for sure if SARS-CoV-2 did originate from bats, we’re likely to be hearing from bats in the future.

When SARS-CoV-2 first hit the spotlight in early 2020, since before a pandemic was even declared, researchers suspected that the virus may have originated in bats. The other likely source are pangolins. A recent 2021 study also found evidence that the virus was circulating in bats and pangolins, but until clear smoking gun evidence is found, the source of the pandemic will remain a mystery and we won’t know for sure exactly where the virus originated from.

But whether or not this new coronavirus emerged from bats or not, several other dangerous viruses made the jump from bats to humans — and it’s very possible that in the future, more will make the jump.

Which begs the question, why bats?

A viral reservoir

The key to the question is the bats’ immune system. Bats have strong immune systems which allows them to host a number of dangerous viruses without actually getting sick.

A recent study found that the key is their ability to limit inflammation. When bats have an infection, they don’t react to it in a way that does damage. For species like humans, the inflammatory response can help fight the infection, but it can also cause damage to the body. Bats’ immune systems can keep viruses under control without inflammation.

“Bats’ natural ability to dampen inflammation caused by stress and infection may be a key mechanism underlying their long lifespans and unique viral reservoir status,” said Dr. Matae Ahn, first author of the study and an MD-PhD candidate of the Emerging Infectious Diseases (EID) Programme at Duke-NUS Medical School.

But other animals aren’t as strong, and if they get exposed to the viruses, they’re ill-equipped to deal with such powerful viruses. Not all viruses can readily make the leap from one species to another — but those that can are often devastating.

“Bats appear to be capable of limiting excessive or inappropriate virus-induced inflammation, which often leads to severe diseases in other infected animals and people,” said Professor Wang Lin-Fa, Director of Duke-NUS’ EID Programme and senior author of the study. “Our finding may provide lessons for controlling human infectious diseases by shifting the focus from the traditional specific anti-pathogen approach to the broader anti-disease approach successfully adopted by bats.”

Because of the way they can handle viruses, bats also act as a sort of training camp for viruses. A separate study on bat cells found that their strong immune response can drive viruses to greater virulence. It’s like an arms race between two armies (the bats and their viruses), with each of them getting stronger and stronger. If a third party (say, another species) is brought into the fray — one that hasn’t been a part of the arms race — they’ll likely be severely outgunned.

“Some bats are able to mount this robust antiviral response, but also balance it with an anti-inflammation response,” said Cara Brook, a postdoctoral Miller Fellow at UC Berkeley and the first author of the study. “Our immune system would generate widespread inflammation if attempting this same antiviral strategy. But bats appear uniquely suited to avoiding the threat of immunopathology.”

This is what happens with spillover events, and it’s very possibly what caused the pandemic.

Coronaviruses are especially problematic

This isn’t exactly news to researchers. In fact, an earlier 2019 study predicted a high likelihood of a coronavirus emerging from bats, and several more may be lined up.

Several types of bat viruses are concerning, but as several studies have suggested, coronavirus are particularly problematic.

We already know that there are hundreds of coronaviruses in bats that could make the jump to other species and humans. Even if bats aren’t closely related to humans, one pathogen could jump to an intermediary species (like pigs, for instance), and from there jump to humans.

The bats’ lifestyle is also ideal for spreading viruses: they spend most of their time in colonies, close to each other, where viruses can spread with ease. They also fly from place to place, spreading their pathogens across a relatively large habitat. Their vigorous flight also keeps them fit.

But concerningly, researchers have recently found that spillovers directly from bats to humans may not be as rare as once thought.

“The bottom line is that bats are potentially special when it comes to hosting viruses,” said Mike Boots, a disease ecologist and UC Berkeley professor of integrative biology. “It is not random that a lot of these viruses are coming from bats. Bats are not even that closely related to us, so we would not expect them to host many human viruses. But this work demonstrates how bat immune systems could drive the virulence that overcomes this.”

Not the bats’ fault

Of course, this doesn’t mean that the bats are to blame — it’s our interaction with them that causes problems.

In places like southern China or Vietnam, there are scores of caves, and bats inhabit most of them. As human activity expands more and more, it starts chipping into bat habitats. It’s not so much that bats are coming to humans and spreading viruses — it’s more that humans are moving closer to bats and contracting viruses.

In addition to the direct interaction, putting bats under stress can further exacerbate the problem, by causing them shed more virus through saliva or urine.

“Heightened environmental threats to bats may add to the threat of zoonosis,” said Brook, who works with a bat monitoring program funded by DARPA (the U.S. Defense Advanced Research Projects Agency) that is currently underway in Madagascar, Bangladesh, Ghana and Australia. The project, Bat One Health, explores the link between loss of bat habitat and the spillover of bat viruses into other animals and humans.

This isn’t restricted to bats. Our interaction with wildlife, in general, is another reason why zoonotic diseases (potentially ones that can cause pandemics) are on the rise. We eat more meat than ever and we’re in contact with more wildlife than ever, and destroying more habitats than ever.

Wildlife trade is also on the rise, and the pandemic has done little to quench its surge. It’s probably not a coincidence that pandemics are also on the rise.

Yes, bats have insane immune systems that make them viral reservoirs. Yes, bats can spill over numerous viruses to us and other animals. But this was also the case one century ago. It was also the case a thousand years ago. This hasn’t changed. What has changed is us — and the way we interact with wildlife. There’s little change that this will improve anytime soon, and until that happens, we’ll likely continue hearing about bat viruses for many years to come.