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From an ecological perspective, consider this.

Humanity is now one of the most successful species on the planet. Seven billion humans is an enormous biomass, and that our livestock make up most of the mammalian biomass on the planet. [1]

So that alone makes us a big juicy target for diseases and parasites which will prey upon us.

A communicable disease which targets, for example, bobcats, isn't going to spread very far or fast, given their low population and disjointed territory.

[1] http://www.kalaharilionresearch.org/2015/01/16/human-vs-live...