We, humans, go into those wild ecosystems.
We cut down trees. We build mines, roads and villages. We kill the animals and eat them. Or we capture them and transport them around the world.
In doing that, we expose ourselves to all these viruses living around the world. That gives the viruses the opportunity to spill over into humans. Then in some cases, once the virus makes that first spillover, it discovers that it might be highly transmissible in humans. Then you might have an epidemic or a pandemic.
via The Ebola Outbreak: 'A Dress Rehearsal For The Next Big One' : Goats and Soda : NPR.