Abstract
When you think of bats, you might picture mysterious creatures flying around inside caves and drinking blood in the night, but bats are much more than that! Did you know that bats are the only mammals that can fly? Not all bats are nocturnal—some are active during the day; some live in trees instead of caves; and they might eat fruit or insects. Bats also protect the health of the environment. They pollinate plants, spread seeds, and eat pest insects that can spread pathogens to crops. Even more fascinating, bats can carry viruses that make people sick, yet they rarely get sick themselves. Could it be that bats do not ever get sick, or do they have special ways of handling infection? In this article, you will explore the fascinating world of bats, their relationship with viruses, and their extraordinary immune systems!
What is Up With Bats and Viruses?
There are many species of bats, and they have lots of fantastic abilities and characteristics that make them very interesting! Bats are flying mammals with diverse behavior and diets (see Box 1 for more bat facts). Bats are also known for carrying many kinds of viruses, with recent estimates numbering in the thousands [2]. Well-known bat–virus associations include: Mexican free-tailed bats and rabies virus; Egyptian fruit bats and Marburg virus; and gray-headed flying foxes and Hendra virus (Figure 1). These three viruses and others can cause life-threatening disease in humans and animals, but bats themselves do not appear to be sick when they have them.
Box 1 - Bat Facts!
Bats are the only flying mammals. Even though they have wings, they are more closely related to humans and other mammals than they are to birds. Like other mammals, bats are warm blooded, give birth to live young, and have fur instead of feathers. The word “bat” refers to a huge group of species, not just one. There are over 1,500 species of bats, all in the same group, called Chiroptera [1]. This means that more than one in five mammal species is a bat—there are more bat species in the world than species of cats, dogs, primates, deer, shrews, and whales combined.
Do all bats live in caves and come out at night to eat blood? No! While this is true of some bat species, such as vampire bats, different bats have incredibly different lifestyles. Some are diurnal (active during the day), and some live in trees or old buildings. Bat species vary in what they eat, from fruit to leaves to insects to small animals (some bats even eat fish). They range in size from bumblebee bats the size of your thumb to “flying foxes” with wingspans taller than some kids! Some species live with only a few other individuals, while others have colonies of hundreds or even thousands of bats. Bats can be found almost everywhere in the world, except for very cold regions close to the poles.
Conservation is especially important for bats because of their roles in supporting healthy environments. They are very sensitive to environmental changes and can help people understand if ecosystems are in danger. Bats further support healthy environments by acting as pollinators, providing fertilizer that is useful for growing food, and consuming a ton of pesky insects.
- Figure 1 - Well-known relationships between bats and viruses.
- (A) Mexican free-tailed bats and rabies virus. (B) Egyptian fruit bats and Marburg virus. (C) Gray-headed flying foxes and Hendra virus. [Photo credit: Virus icons are from BioRender.com; Mexican free-tailed bats, Meagan Allira; Egyptian rousette, Andrew Mercer, Wikimedia Commons (https://commons.wikimedia.org/w/index.php?curid=81590925), gray-headed flying fox, MgeniNL, Wikimedia Commons (https://commons.wikimedia.org/w/index.php?curid=153402954)].
If bats have so many viruses, why do they not get sick? The short answer is that scientists do not know. The longer answer is that there are a few interesting hypotheses (educated guesses) about what makes bats great carriers, also called hosts, of viruses.
Maybe Bats Do Not Get Sick Because They Fly?
Flying is physically demanding; bats can fly long distances, even while carrying their young. Female bats carry the equivalent weight to a human walking (or flying!) around carrying an 8-year-old child! To deal with such high demands of long-distance flight, bats have developed specialized muscles, high lung capacities, and fast metabolisms. Flight may also impact the function of the bat immune system (the bodily system animals use to handle infection) in ways that likely shaped their relationship with viruses.
Like humans, bats have innate and adaptive immune systems. The innate immune system is the body’s first line of defense when damage occurs. When you get a paper cut, you might notice the area become red, hot, and swollen. This response is called inflammation, where many innate immune cells rush to the impacted area to attack invaders (called pathogens) that come through the cut in your skin. Innate immune cells use general tactics like trapping and eating invaders. The adaptive immune system brings in specialized cells as backup to the innate immune cells. These cells can target specific invaders and help repair damage. Inflammation decreases once the invaders and damage are taken care of, but what happens if inflammation does not go away? Uncontrolled and long-term inflammation (called chronic inflammation) can be dangerous, because the immune system is still attacking after the threat has passed, leading to tissue and organ damage.
A unique feature of the bat immune system is its ability to avoid chronic inflammation while flying. When in flight, bats work their muscles very hard and burn a lot of energy. All that burned energy makes toxic byproducts that can damage DNA and cause harmful mutations (Figure 2)—the more energy burned, the more harmful molecules. Human immune systems would mistake this for a big threat, triggering intense inflammation (i.e., a flood of immune cells into damaged areas of the body). Instead, bats “dial down” inflammation with helpers that keep immune cells “in line” to prevent an overreaction but still allow the DNA damage to be repaired. The benefit of this dialing down on inflammation is that bats can reduce the cell and tissue damage caused by viruses, allowing them to tolerate being infected.
- Figure 2 - Bats have unique responses to inflammation because they need to control inflammation caused by sustained flight.
- (A) When bats fly, toxic byproducts are generated from expending too much energy. These byproducts damage cells and DNA. (B) In humans, this amount of damage would send a flood of immune cells to the damaged area and cause intense inflammation. (C) Bats use helpers to prevent this intense inflammation, by slowing down the rush of immune cells. This way, bats can sustain flight without muscle pain (Illustration: Briana Betke).
Bats and Viruses: The Ultimate Long-Term Relationship
Bats might be able to handle viral infections better than other species because they have had a long time to get used to living with these viruses.
Surprisingly, the relationship between bats and some viruses is even older than that of viruses and humans. Bats are older than modern humans by over 50 million years [3]. If all our history could fit into a year and bats and viruses began interacting on January 1st, humans would get their first virus at about 8 p.m. on December 28th! Although many bat–virus relationships have existed for millions of years, only a small portion of these viruses are known to make humans sick. Importantly, any given virus in a bat is not any more likely to infect humans than viruses found in other mammals or birds [4].
How have bats and viruses coexisted for so long? Bats and viruses are locked in a cycle of trying to best one another, each creating a new defense against the other to survive (Figure 3). For example, viruses have “keys” that can open the “doors” into bat cells, leading to infection. In response, the bat cell changes the lock and prevents entry of the virus. The virus then makes a new key and can enter the bat cell again. Over many years, this back-and-forth between the bat cells and viruses leads to viruses that are very skilled at overcoming bat defenses and bats that are very good at surviving while carrying viruses. When two or more species spend so much time together that they evolve together over thousands or millions of years, this is called coevolution. The process in which bats and viruses are constantly one-upping their defenses against each other to survive is a special type of coevolution called a coevolutionary arms race.
- Figure 3 - Bats constantly create ways to keep viruses from entering their cells and causing infection.
- In response, viruses find ways to get around bats’ defenses. (A) A virus encounters a bat cell. Since the virus has a “key” that matches the lock of the bat cell, the virus can unlock the “door” and infect the bat cell. (B) The bat cell creates a new lock to prevent the virus from entering. (C) Eventually the virus can make another key to enter the bat cell again. (D) The bat cell creates a new lock again to prevent being infected by the virus. This back and forth, known as a coevolutionary arms race, continues. After thousands (or even millions) of years, this process leads to viruses that are good at infecting bats and bats that are good at tolerating viruses! (Illustration: Mika O’Shea).
Wait, Do Bats Never Get Sick?
Bats are not invincible. They can, and do, get sick and die from viruses. For example, like all mammals, bats get sick and die from rabies and closely related viruses, all of which are transmitted through the bite of an infected animal. Bats are also vulnerable to Tacaribe virus (which is not related to rabies and not known to make humans sick), in addition to other types of pathogens like bacteria and fungi. The fungus that causes white-nose syndrome has killed millions of North American bats during their winter hibernations.
Even if infections do not directly kill bats, they can still pose major problems. Sick bats may starve if they cannot travel to find food or successfully hunt insects, potentially impacting their ability to support healthy environments. Sick bats may also not be able to move to new areas if humans change their habitats. Even bats that survive infection may not be strong enough to have and care for their babies, which endangers the species.
So Much We Do Not Know Yet!
Many questions remain about how bats limit getting sick from viruses. Understanding how bats stay healthy will help scientists balance limiting the spread of harmful viruses while protecting bat populations and allowing the environment to benefit from the many services bats have to offer. Here are some of the questions bat researchers are still actively investigating.
What does it mean for a bat to be “sick”? While scientists have an in-depth understanding of what sickness looks like in humans and many domestic animals (such as dogs, cows, and cats), understanding sickness is more complicated for bats and other wildlife. Rather than simply developing a cough or nausea like humans, bats might stop eating or distance themselves from their roosts. They may even show no outward signs of sickness at all. Researchers need to develop better tools to study and define health for bats and other wildlife so that they can protect bat populations.
How do bats’ adaptive immune systems protect them from infection effects beyond inflammation? Some studies suggest that bats, compared with humans and other animals, do not produce many antibodies (the proteins responsible for long-term immunity). Further research is needed to understand how bats can protect themselves from specific pathogens without relying on antibodies.
Do bats’ adaptive immune systems produce fewer antibodies because their innate immunity is so strong? Scientists are investigating this possibility while also exploring how bats’ innate and adaptive immunity work together to strike the perfect balance during viral infection.
How much do these qualities vary between different groups of bats? Most of the research on bat immune responses to viruses has focused only on a handful of species. With a family tree made up of over 1,500 bat species and spanning over 50 million years of evolution, there may be a lot of variation in their unique immune qualities [5].
Understanding Bats Means Better Health For All
Bats are fascinating creatures that, while not protected from all pathogens, carry many viruses without appearing to be sick. This ability may be due to how they reduce inflammation caused by flight and their long relationships with viruses, allowing bats to refine their strategies to prevent damage from viruses. However, these explanations are educated guesses and many questions about bat immune systems are unanswered. Answering these questions would provide a clearer picture of how bats handle infection from viruses, informing conservation efforts, public health efforts, and development of medical technologies to manage chronic inflammation in humans.
Glossary
Chiroptera: ↑ The group of organisms that contain all bat species. It means “hand wing” in Greek, referring to the structure of their wings, where skin connects their “fingers” and “thumbs”.
Immune System: ↑ The group of cells and organs that fight off infections and injuries. It is composed of innate and adaptive parts.
Innate Immune System: ↑ The body’s first line of defense against intruders. It responds quicker than the adaptive immune system.
Inflammation: ↑ Part of the immune system’s response to injury, illness, and anything that does not belong in your body (such as pathogens). It generally involves swelling and redness of impacted tissues.
Pathogen: ↑ An organism that survives and reproduces by making the organism it invades sick. Examples: viruses, bacteria, fungi, and parasites.
Adaptive Immune System: ↑ The branch of the immune system that fights using helpers that are specifically suited to each invader. It reacts more slowly than the innate immune system.
Coevolution: ↑ When two or more species adapt together (i.e., evolve) in response to environmental changes over a long period of time.
Coevolutionary Arms Race: ↑ A special case of coevolution where a species develops a defense and another species creates a counter defense in response.
Acknowledgments
The authors were supported by funding to the Viral Emergence Research Initiative (VERENA) Institute, including NSF DBI 2515340. Additional support was provided to BB through the NSF PRFB (DBI 2305782).
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Conflict of Interest
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References
[1] ↑ Simmons, N. B. and Cirranello, A. L. 2024. Bat Species of the World: A taxonomic and geographic database. Version 1.6.
[2] ↑ Letko, M., Seifert, S. N., Olival, K. J., Plowright, R. K., and Munster, V. J. 2020. Bat-borne virus diversity, spillover and emergence. Nat. Rev. Microbiol.. 18:461–71. doi: 10.1038/s41579-020-0394-z
[3] ↑ Simmons, N. B., Seymour, K. L., Habersetzer, J., and Gunnell, G. F. 2008. Primitive Early Eocene bat from Wyoming and the evolution of flight and echolocation. Nature. 451:818–21. doi: 10.1038/nature06549
[4] ↑ Mollentze, N. and Streicker, D. G. 2020. Viral zoonotic risk is homogenous among taxonomic orders of mammalian and avian reservoir hosts. Proc. Natl. Acad. Sci. USA. 117:9423–30. doi: 10.1073/pnas.1919176117
[5] ↑ Becker, D. J., Vicente-Santos, A., Reers, A. B., Ansil, B. R., O’Shea, M., Cummings, C. A., et al. 2025. Diverse hosts, diverse immune systems: Evolutionary variation in bat immunology. Ann. N. Y. Acad. Sci. 1550:151–72. doi: 10.1111/nyas.15395