It’s rather arrogant as a species for us to believe that enough evidence remains tens of millions of years after something like that happened that we can come to a conclusion. For all we know, a pandemic could have swept across the globe and killed all the dinosaurs and there would be no way for us to detect it.
1. Such a pandemic (or other calamity) occurring simultaneously with overwhelming evidence a catastrophic asteroid impact is exceedingly unlikely.
2. Cross-species pandemic resulting in mass extinctions are similarly unlikely. Boom-bust population cycles perhaps. Mass cross-species extinction not so much.
3. Patterns of extinction (and survival) far more closely match the presently-accepted conclusion.
4. Precisely because an infectious agent leaves few multimillion-year traces, any defence of such a hypothesis can advance no further than "it might have happened". This is not evidence-based argument. (Traces of pathology might be preserved in fossil or amber substrates.)
Point remains that multiple highly consistent widely-distributed lines of evidence point to the asteroid hypothesis.
One unsubstantiated Hacker News comment points to the epidemic scenario.
