Parasitism is the most popular lifestyle on earth – over 40% of all known species are parasitic, and the number of parasitic species rises daily.
Though they’re largely ignored when we study food webs, parasites have been estimated to be involved in over 75% of inter-species interactions. By the numbers, they are the most prolific and successful organisms on earth.
[Via Scientific American]
Science-Based Life 
Check out “Parasite Rex” – its a great read. I got into it when I had Giardia.