Nobody knows how many species live on the Earth
and how many of them are parasitic. (Peter W. Price
(Evolutionary ecology of parasites, Princeton
University Press, 1980) has suggested that there
are more parasitic species than free living ones.
According to another estimate, mentioned by Zimmer,
80% of all the species are parasitic.
Most parasites control behavior of their hosts in
many ways. They cull some hosts, benefit other hosts,
and stabilize ecosystems. They are the managers of
nature and essential components of biodiversity.
Zimmer noticed that humans are parasites of Gaia,
but did not mention parasitic relationships between
humans. My personal experience indicates that human
parasites are just as common as the wild ones. The
abundance and importance of parasites makes a mockery
out of our lofty ideas about nature, God, and humans.
If we exterminate parasites, we will exterminate our
world. If we do not exterminate them, we will hate
our world, and drift towards nihilism, narcotics,
and suicide. Fermi Paradox suggests that this is the
choice of all advanced civilizations. The only way
to enjoy our world is to replace our ethics of
justice with a new ethics based on diversity:
http://www.islandone.org/LEOBiblio/DIVERSIT.HTM.
I'm not sure it's this article your citing, but
somewhere I've read that around 1% of the human
body mass is parasite biomass. Considering that
another 4/5 is H2O humans don't have that much
living room...