Economics

People have the ability to choose to not damage "the host"?

So we do it willingly?
"Many" when talking for a species is meaningless.

Some brain cancers might heighten some of the brains abilities ... yet I don't think that matters.

Also which humans don't negatively affect kilometres of Earth's surfaces and species for 100s of thousands of years?

Beavers, or any species really, can and do affect experientially all they can. They do that until they are in equilibrium with the ecosystem. Invasive species are perhaps a more clear example of this process.

The relative speed of the process and how fast the environment responds is crucial in the infestation definition.

In any population the initial growth is basically limited only by the resource availability. So any species at some point, especially at the beginning, behaves (and it's evolutionary beneficial to do so) like an infestation, the limits come from the environment, and in complex environment that means other species. That's how ecosystem grow from single species to complex interaction between 1000s of species in more or less stable equilibrium.