Over 1,500 bat species carry thousands of deadly viruses but rarely get sick, and scientists are only just beginning to understand why

Bats are one of the most misunderstood and fascinating animals on the planet. To most Americans, they're something out of a Halloween decoration, creepy, cave-dwelling, blood-sucking. But scientists have been quietly obsessed with bats for a whole different reason: these animals are home to some of the deadliest viruses known to man, including Ebola, Marburg, Nipah, and very likely the original strain of SARS-CoV-2, and yet they walk away totally fine. What is happening inside a bat’s body that we just don’t have?

In 2024, researchers at MIT’s Whitehead Institute published a study in the Proceedings of the National Academy of Sciences that found bat cells have a uniquely preactivated innate immune defense that stops a virus from fully replicating – even after it’s already entered the cell. In experiments, SARS-CoV-2 infected bat fibroblasts but failed to produce new viral particles. The study's lead author, Rudolf Jaenisch, a professor of biology at MIT, noted that bat cells have “elevated expressions of antiviral genes that act immediately, neutralizing the virus before it can spread.” In other words, bats don’t wait for the virus to make them sick to mount a defense. They are already armed.

The secret is in how bats handle inflammation
When most mammals, humans included, encounter a virus, the immune system mounts a full-blown inflammatory attack. This is usually helpful. The problem is that the system can overshoot. Consider the severe cases of COVID-19 where the patients were suffering not only from the virus but also from their immune system going haywire. That type of runaway inflammation causes organ damage, destroys tissue, and, in severe cases, kills.


Bats seem to have solved this problem. In a study published in Cell Host & Microbe, virologist Peng Zhou discovered that bats deliberately suppress an antiviral immune pathway known as the STING-interferon pathway. So instead of a huge immune response when a virus shows up, bats keep just enough immune activity to get a handle on the virus without doing collateral damage to their own bodies. It’s a “balance between bats and the pathogens they carry,” Zhou said, pointing out it’s been perfected through millions of years of evolution.

This restraint is believed to be connected with flight. Bats are the only mammals capable of flight, and sustained powered flight is one of the most physically demanding activities in the animal kingdom. It produces lots of metabolic waste, including molecules that can damage DNA. Most animals would have a bad case of inflammation with that level of cell damage. Bats evolved to suppress that response to protect themselves in flight and, as a side effect, that same suppression also protects them from viral overreaction.
50 million years of practice
And one of the things that makes bats so unusually good at tolerating viruses isn’t just their immune system. It’s time. In research published in the journal Evolution on bat-virus coevolution, scientists say the oldest known bat fossil is about 52.5 million years old, meaning bats and viruses have been in an evolutionary arms race for an extraordinarily long time. The viruses evolved better ways to get into bat cells; the bats evolved better defenses in turn. That cycle, again and again, has created bats that are highly resilient hosts and viruses that are highly skilled at living inside them.

This coevolution also sheds light on a common misconception that bats are uniquely dangerous carriers of viruses. The chance of a virus in bats infecting humans is no higher than the chance of a virus in other mammals or birds, studies have found. It's not that bats are more dangerous than any other wild species. It's that habitat encroachment, the wildlife trade, and more human contact with animals that cause viruses to jump to humans.

What bats can teach us about our own health
A University of Saskatchewan study examining how bats carry the MERS coronavirus without getting sick found that the bat immune system doesn’t shut down like human immune systems do when confronted with the virus. “The bats don’t get rid of the virus and yet don’t get sick,” said microbiologist Vikram Misra, who headed the study. That mechanism, he said, could ultimately help explain how coronaviruses jump to people in the first place.

Scientists say cracking the bat immune code could have real implications for human medicine, especially for diseases involving chronic inflammation, such as autoimmune diseases, long COVID and even aging. If we can understand how bats are able to tolerate viral infection without killing themselves, we may be able to develop therapies that help humans do the same.

Bats are not invincible, and they need protection
It is worth remembering that bats are not immune to everything. They can and do die of rabies, and a fungal disease known as white-nose syndrome has killed more than 6 million North American bats since it was first detected in a cave in upstate New York in 2006. Habitat loss, as well as pesticides and climate change, are also driving several species to extinction.

And it matters not just for bats but for us. Bats pollinate plants, spread seeds, and eat huge numbers of insects, including agricultural pests that would otherwise destroy crops. The loss of bats is not just a problem for ecosystems; it’s a problem for the food chain and public health that spreads outwards.

Scientists are still trying to piece together the full picture of bat immunity. But what is already clear is that these animals we have feared for centuries could hold some of the most valuable secrets of medicine.