I'm sceptical, but not entirely disbelieving of this. The humans wouldn't necessarily have had contact with all the animals. Just infect a few, who roam within their habitats, and infect others. Just like the Spaniards didn't travel all the way into the Continental US, but smallpox certainly did via trade routes. Good book on this subject btw...
Armies of PestilenceOver 100 specie of flora and fauna go extinct everyday.
Actually, Spaniards did penetrate the Continental US. We had them all the way up into central-eastern Oregon, on gold explorations. They also made pretty secure footholds in the Southwestern US areas, and certainly in California... and it was all done fairly early on (1700's for Oregon)
Regards,