According to Dictionary.com The definition of plague is -
Looking at the third and fourth definitions, I might conclude that humans are indeed a plague upon the planet and the planet would doubtless be better off without humans. However, presumably as a human (I just took a quiz that claims I am an alien), I find this a tad disturbing. Like any species, humans exploit their environment to satisfy their needs. With humans being a dominant species with an extremely large biomass, we can not help but foul our environment to some degree. I think it is a matter of perspective and the criteria used to make such a judgement. Then the thought occurred to me as to whether humans have ever done anything that has actually benefited the planet?