When NASA spotted a derelict spacecraft on the outer edge of Earth's atmosphere, they had no idea where it came from. A salvage mission was launched, and relic was retrieved and brought back to earth. No one knew the power of the relic, or that it would release deadly spores into the atmosphere.
The plague that resulted from the spores spread worldwide. Those infected found out, death is no escape. Those not infected, lived witnessed the new breed emerge called Walkers.