Many little parasites add up to one big biomass - earth - 24 July 2008 - New Scientist Environment

The main point of the article is that parasites are much more populous than previously believed. But this quote was the most interesting:

Many little parasites add up to one big biomass - earth - 24 July 2008 - New Scientist Environment:

"Dobson says he has work in press showing that parasites contain between 10 and 1000 times more pollutants than their hosts. He suggests parasites might play an important role in cleaning up the animals they inhabit."

So they castrate you, but they also keep you clean...