Why bats can fight off so many viruses

• Many animals are capable of transmitting viruses to others, but as virus super hosts, bats are common transmitters.
• Scientists believe the COVID-19 coronavirus could have originated in bats.
• As a by-product to flying, bats' weakened immune systems have given them the ability to live with viruses, without displaying serious symptoms or catching a disease.
• As humans encroach on wild environments, the opportunities for transmission, as wildlife directly interacts with people and domesticated animals, have increased.
• For the latest case total, death toll, and travel information, see Business Insider's live updates here.
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Following is a transcript of the video.

Narrator: Ebola, rabies, SARS, and COVID-19 are not just viral diseases. They are zoonotic diseases, caused by a virus transmitted from animals to humans. While many animals are capable of transmission, there's one in particular that is considered a virus superhost: bats. So, what makes Dracula's alter ego also the perfect breeding ground for viruses?

Bats harbor significantly more zoonotic viruses compared to other mammals. Researchers have found more than 60 of these floating around in the bat population. And, on average, each bat plays host to almost two of these viruses. Some of these are linked to large-scale outbreaks, such as MERS, SARS, and Ebola, which were spread to humans either directly from bats or through intermediate hosts. And scientists have suggested that bats could also be linked to the COVID-19 coronavirus.

But with such a heavy viral cargo, how do bats manage to survive? Researchers believe it actually comes down to their ability to fly.

Jon Epstein: The inflammation that results from the physical act of beating your wings, you know, hundreds of times a minute, that can lead to damage at the cellular level.

Narrator: That intense damage would typically be enough to trigger an immune response, the same reaction your body would have to an infection. This includes symptoms like inflammation, a fever, or even mucus production, and, for the most part, this reaction is to protect the body. But sometimes immune systems overreact, which can trigger severe diseases such as pneumonia, as seen with COVID-19 cases. So the aggressive and consistent amount of damage caused by flying has forced bats to adapt.

Epstein: Bats have dampened down their response, their inflammatory response.

Narrator: They produce only enough of a response to survive and fly without the body going into overdrive. Basically, without causing disease. And that response to flying comes with a rather helpful byproduct: the power to fight viruses. Their weakened anti-inflammatory response not only stops the body from going into overdrive, it also allows them to always be on high alert for viruses.

Normally, mammals have to switch off their antiviral response systems to avoid inflammation, but scientists have found that some bats never switch off their antiviral response at all. Their immune system is basically always on the lookout for a new viral invasion, so when a virus attacks, bats can swiftly protect their bodies against it. They don't necessarily kill the virus, but they do protect their cells against it, essentially acting as viral incubators, coexisting and tolerating it just enough to survive.

Epstein: Most natural reservoirs have a long-term relationship with viruses and bacteria. They've spent a long time together, and often infection with those microbes is benign in its host, so it's really the interaction with new hosts, the ability for viruses to get into a new animal or people that leads to disease.

Narrator: So while all these viruses might be deadly to other mammals, bats themselves can carry them for a period of time without having any serious symptoms, and during that time, they pass it on. Bats make up about 20% of mammalian species, and they also love being together, which enhances the spread of viruses between them. And, well, they get around. The sky, that is. Bats can travel wide geographic distances in very little time. This means they also pass their, uh, viral fluids to a wide range of animals, and these animals can pass them to humans through saliva, feces, or urine, oftentimes when animals are being handled, hunted, butchered, or eaten. This is exactly what happened in 2002 when a horseshoe bat infected a civet with SARS, which then passed on the disease to human populations. But before we all start a bat witch hunt, let's take a step back. Bats are essential to our society and ecosystems. They control insect populations, pollinate crops, and teach us about sonar systems.

Epstein: So, we wouldn't have our tropical rain forests if it weren't for bats. So these are animals that are critically important. So even though we recognize that they may carry viruses that have the ability to infect people, we have to focus on human activities and the things that we're doing that bring us into contact with bats, because the spillover is typically accidental, and it's caused by us.

Narrator: Currently, humanity is doing more and more each day to bring us into closer contact with wildlife. We're expanding cities and farms into wild areas, tearing down forests, and capturing animals for wildlife markets or illegal trading. These all increase the chances of wildlife directly interacting with people and domesticated animals, and that can lead to the emergence of new viruses in human populations. COVID-19 isn't the first and most certainly won't be the last zoonotic-disease pandemic, but we can be better prepared in the future.

Epstein: We need to continue to put resources into public-health measures, strengthening health systems in countries that are most vulnerable to emerging diseases and continuing to understand what viruses are out there that are potentially risky for human health, and what are the activities, what are the actions going on that put us at risk so that we can work with local communities and with countries, including the United States, to try to reduce high-risk behaviors.

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