Y'know the more I think about the lone tribble on Lorca's desk and how several people thought it was a stupid thing to have on a ship (because where there's one tribble, there's a million), something hit me this morning as I was driving into work.
What if tribbles were not always like we know them? What if they had a reasonably long development and gestation period and, somehow, Section 31 (whose involvement here is pretty much a foregone conclusion at this point) had a hand in genetically re-engineering these things to turn them into ecology-destroying eating/breeding machines? Think about it - they're probably pretty simple organisms compared to humanoid life, with a relatively simple genetic structure. We can already manipulate such things in plant and animal life forms now. In the Trek universe, with a mastery of genetic manipulation at the human level creating Khan and the other Augments in the late 20th century, imagine what a group like S31 with unlimited black-source resources and a flexible moral compass could accomplish 2-300 years from now? Lorca and his crew are clearly doing bio-genetic experimentation on a vast number of life-forms, looking for new and innovative ways to win the war against the Klingons by any means necessary (a known S31 mantra). Saru said they could do, what, over 200 individual science experiments on Discovery? Yup...
I am now convinced that the tribbles were a genetically-modified S31 bio-weapon specifically designed to attack Klingon worlds, which they effectively did, per Worf's screed about them in DS9. They're cute and fuzzy, so most humans and likely many other Federation beings who encounter them wouldn't really think to consider them dangerous in any way. McCoy was, at heart, a good guy with a strong moral foundation and likely didn't have much of a grasp of the conspirator's mind. He probably didn't even think to dig too deep into their genetic structure to find artificial tampering when he did his analysis on them.
It really, feels like it makes an immense amount of sense - I joked earlier in the thread that Lorca had the tribble "fixed" to prevent it from breeding and uncontrollably overrunning the ship.
What if his "pet" was actually patient zero?