Man cannot necessarily be described as a virus to the earth. We more than just feed on the earth. I think if my knowledge of biology is not that rusty, then viruses don't add value to their host. Man on the other hand gives to the earth. In many places and times in history, we have beautified the earth and made it a better planet. In other circumstances, we have also been viruses. So, it's a give and take thing. We uproot trees and harvest plants. But we also plant and nurture the vegetation too. And viruses don't do that.