Around 1110, in England, there was a very strange disease that killed all kinds of mammals and birds. No one knows what it was and it doesn't sound like any known disease. I read about it in an old book I have. Perhaps it was the same disease that caused the last mass extinction.
Yes, but did this disease cause any species to go extinct? That's the proof you'd need to give this theory any credence at all.
Frankly, this theory is bonkers. Cross-species infection is very rare; I've never heard of humans infecting wild, non-domesticated animals. The only wild animals ancient man would have had contact with would have been animals he killed and ate - it's kinda hard to spread a disease from man to wild animal population that way, would you not agree?
Of course domesticated animals sometimes cross-species infect humans (for example domesticated pigs can spread new flu varieties to humans). But humans infecting wild animals populations: I'd need to see demonstrable proof that this has actually happened in the past. Then I'd need to see demonstrable proof that in the past a disease had once wiped out an entire species. Even if there is proof of both, this theory is a long shot at best.