As others have already said, it’s probably very low or even zero. However, if we allow a chain of events to count, then that number should go up. In that case, you could say that an ant was among the many factors that contributed to the death of a human.

Let’s say you’re tired, driving in the middle of the night, it’s raining sleet, your car has worn-out summer tyres, etc. It’s a recipe for a disaster already, but then you realise an ant has sneaked onboard and is now walking up your leg. You swipe it away, get distracted, and hit oncoming traffic. A truck carrying huge steel beams plows through your 1986 Nissan Micra like it’s made of thin cardboard. Did an ant kill you? Kinda, but not really.