The problem, as with most of these questions, is that it’s way to vague to allow any definitive answer.
For example if a person were immobilised and a mouse chewed through the exposed artery in the ankle they would die within minutes. In reality that’s about as likely as a chocolate kettle, but its possible. At the next level a single fire ant could indisputably kill a healthy adult… if it was swallowed and stung the victim on the right part of the throat on the way down. Airway swells shut, victim dies. But does that count? It’s not a highly likely scenario but it’s entirely possible and I wouldn’t be surprised to find out that someone has dies in exactly that manner. After all plenty of people have been killed or come near death from swallowing wasps in that manner.
And the possible scenarios form a continuous spectrum. Someone has already mentioned death from frog poisoning. And an Irukandji could kill a person, but it’s along way from being a certainty. It requires a specific set of circumstances, not least of which is that in reality you need a lack of care so the person drowns.
Which brings me to the next question: what care or negligence is allowable? Do we assume basic first aid or and ICU or a naked person tied down and immobilised with drugs? That will affect the possible answers significantly.
My personal guess would be Clostridium botulinum. It produces and incredibly potent and stable toxin, and it does so indefinitely. If you could isolate a single cell engineered to be non productive it would, given sufficient time, produce enough toxin to kill a person. A single prokaryotic cell can kill most healthy adults under those conditions. But once again we have the problem that the actual scenario isn’t going to occur under normal circumstances.
I could give any number of other strong contenders but they all depend in the exact ground rules.