Why do bats spread so many diseases? They’re evolutionary marvels.

There are way more bats than you might think. Second only to rodents, bats make up around a fifth of all mammals, with over 1,500 species of winged nightflyers. But it’s this wide biodiversity that also contributes to their (somewhat undeserved) reputation as disease carriers. According to biologists, 50 million years of evolutionary adaptation make bats uniquely suited to not only contracting and spreading pathogens, but also resisting them.

“[Infectious disease] has been the biggest factor in all of evolution,” University of Florida veterinary pathologist Jim Wellehan said in a recent profile. “People are always looking for an excuse why bats are magic, and the truth is bats have just been exposed to a lot of stuff and selected for those genes accordingly.”

Wellehan contends the most important factor for an animal population to thrive from an evolutionary standpoint is its genetic diversity. A broad spectrum of variation greatly increases the chance that at least some individual animals will possess immunity genes that are effective against whatever new pathogen strains may come their way. Ideally, these immune creatures survive long enough to reproduce and pass along those valuable genes to the next generation of offspring.

“When I first learned about Darwin and evolution, ‘survival of the fittest,’ I assumed, meant ‘smartest and fastest and strongest,’ but if you look at our genomes, [it] turns out that’s wrong,” explained Wellehan. “The genes that are selected for are mostly immune-related.”

Among bats, these pathogens spread even more because many species live in highly social colonies. Taken together, this means bats pass diseases between one another in tandem with the genes necessary for developing immunity better than most other animals.

But what can make the animals dangerous is also what primarily sets them apart from all other mammals—its wings. Flight has dramatically expanded bats’ habitat range over millions of years, allowing them to traverse humanity’s artificially constructed geopolitical borders without a second thought. This is how disastrous pathogens like Ebola, Nipah virus, and even the SARS-CoV-2 virus that causes COVID-19 can wreak such havoc on society. 

That said, don’t start blaming bats just yet. They’re simply living in a world that is increasingly disrupted by society’s encroachment. This avoidable close contact between humans and bats is what ultimately can cause diseases to leap between species.

“Pathogen transmission to humans and conservation efforts go hand-in-hand. When populations get under stress, that’s when ecological balances get shifted, and zoonotic jumps occur,” said Wellehan. “It turns out that if we think of ourselves as something separate from nature, it doesn’t work so well.”