I answered a similar question earlier on in IAS, and I came to the conclusion that it was likely to be one of the bacterial species. They can breed very quickly and can overcome much larger organisms. I am afraid I couldn’t pick a particular bacterium. I think beetles (many species, resiliant, diverse, widespread) and humans (which are large animals with a very wide range, and a unique -?- culture) would come very close, too.
I hope you won’t mind if I use this as an opportunity to ask for your vote on Friday – if you want to know why, check out my video blog.