Some of the worst outbreaks of viral diseases have their origins in the only flying mammal: the bat.

For a long time, we’ve wondered why bats are the sources of so many viruses, such as Ebola, Marburg, Hendra and SARS. Some new data and modeling revealed that bats have unique immune systems that allow them to carry viruses without getting sick.

Viruses can reproduce quickly in bats, and the special immune system means that the viruses can be more dangerous when they get into people.

Zoonotic diseases are diseases that can jump from animals to humans, and bats carry more than any other animal. Zoonotic diseases from bats are also deadlier for humans than diseases from other animals.

Sustained flight is “physiologically expensive,” not only requiring wings but also a high metabolic rate. Higher rates of metabolism typically result in shorter lifespans because of the production of free radicals. But bats have evolved ways to avoid this damage, and they can live up to 40 years.

Interferon (IFN)-alpha is a protein in our immune systems that tells other cells to get ready for a viral infection. Those cells go into an anti-viral state, which limits viruses from entering and slows viral spread and disease progression. A byproduct of IFN-alpha is inflammation and that achy feeling you get from the flu.

Too much inflammation can cause damage, but not in bats. Some bats make IFN-alpha constantly, leading to a constant antiviral state. This much was known to science, but nobody knew how it resulted in deadly zoonotic diseases.

For this new data, scientists put viruses in bat cells that create IFN-alpha at different levels. They then used a computer to model how the viruses spread in bats and how the bats’ bodies responded. They then used simulations to determine how well the models fit the data.

The bats’ hyperactive antiviral systems protect them from getting sick, but it also means they have long-term infections. The scientists learned that viruses stick around in the bats’ bodies, reproducing quickly. As these viruses reproduce, they start to mutate. Some of these mutated viruses become dangerous, and some can also infect other mammals, including humans. Just think about the SARS and Ebola outbreaks we’ve experienced.

We’ve seen several viruses jump from bats to humans, sometimes leading to large outbreaks and many deaths. These results help to explain why bats are the source of such nasty viruses when humans get infected.

Understanding the role of bats will be important in controlling future outbreaks. This work is based on cells grown in the lab and computer modeling, so we need to do more work in live bats to fully understand the dynamics of viruses in these animals.

It’s also important to remember that bats are a diverse group, with more than 1,200 different species. Many of these species don’t spread viruses, especially in North America. You don’t need to be afraid when you see them on summer nights, flying around and eating bugs.

Medical Discovery News is hosted by professors Norbert Herzog at Quinnipiac University, and David Niesel of the University of Texas Medical Branch. Learn more at

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