Humans are viruses. It is that simple.
Viruses feed off the material of a host body, draining it, and using it to sustain itself for growth and development.
We are the same. Stripping our planet of its resources we grow as a civilisation, advancing our technology and evolving along with it.
Our cities and buildings are like the sores and wounds that erupt from the flesh when the host's body reacts to the parasite feeding off of it. Our structures are an eye-sore on a landscape torn by the work of our construction machines. The pus that spews from the host are like the fumes that our factories release into the air, polluting it.
When a virus attacks the body, it heats up, causing a fever, attempting to destroy the parasite. This is similar to the Global Warming that is affecting the Earth at the moment. Nature is attempting to destroy us. We, are the parasites now.
A virus survives by infecting other bodies when its first host dies from the work of the parasite. Likewise, for Humanity to survive after we have completely turned this planet into a barren wasteland, we would need to abandon it and search for new worlds to survive on.
We are contagious. We are the viruses, never-ceasing to devour whatever we come into contact with, ever-taking, rarely contributing.
We are the plague, the epidemic, which would inflict a thousand worlds in the future with our scum.
We, then, are no better than the diseases which we fight.