It’s never been easier for animal pathogens to spill over into humans.

Over the past 40 years, disease outbreaks among humans have become more frequent. The majority of those diseases are zoonoses, or diseases that originated in animals, like Ebola, West Nile virus, and probably Covid-19. But what makes zoonotic outbreaks likelier than ever is actually something humans are doing.

Science journalist Sonia Shah, author of the 2017 book Pandemic, says the expansion of humans onto more of the planet’s land has increased the likelihood of disease outbreaks in two ways. First, as humans move into what were once animal habitats, we end up living closer to animals that might contain dangerous pathogens. Secondly, as we destroy or alter animal habitats, we’re driving away or killing off animals that once served as a “firewall” between those pathogens and us.

And the human land development