I'll disclose a secret: I do not believe that "an organism as a whole has one function". I don't believe that an organism as a whole has any function at all! I believe that reproductive success is the only selection factor in evolution, but calling it "function" was a stretch in the sake of analogy with TRIZ.
It is not obvious at all that "the ratio (useful effects)/(harmful effects) is definitely higher for humans than for viruses." Actually it seems quite opposite. Many would disagree with your definition of "usefulness" and "harmfulness" regarding both viruses and humans. There is a very long list of harmful effects from the human reproduction success. Some extreme groups even call humans "viruses on Earth". Some non-extreme groups theorize that humans might be a cause of an ongoing mass extinction similar to the one 300 million years ago, that caused elimination of estimated 95% species on Earth existed prior to it.