For more than a decade, scientists have been sounding the alarm about the increased risk of a pandemic caused by viruses that jump species from wild animals into humans. Most scientists believe the interaction between people and wild animals led to the Covid-19 pandemic, and they say the pandemic shows that we haven’t learned the lessons of past outbreaks.

Cross-species diseases have existed for hundreds of years, and are thought to be the cause of several modern disease outbreaks, including HIV, SARS and SARS-CoV-2, the new coronavirus. With SARS, “we saw the emergence of a new coronavirus thought to be from an animal,” Dr. David Heymann, a public health expert who led the global response to SARS at the World Health Organization, told us. “That was a wakeup call that things could spread rapidly around the world.”

As the human population expands, clearing rainforests, trading in wild animals, and otherwise damaging the environment, the number of these so-called “spillovers” appears to be increasing.

“What we’ve seen now is an acceleration of emerging infectious diseases,” Jon Epstein, a disease ecologist, told us. “There have been more and more outbreaks over the past few decades.”

Here, we look back at the lessons of earlier outbreaks, speaking with the scientists who venture into rainforests and bat caves to understand how viruses hop from animals to people, and how to stop the next pandemic.