I came across this and thought it was a very informative piece. Parasites are any organism which uses a host for their own survival. Although we have bacteria, fungi, etc. on and in us at all times, it's usually symbiotic which means it is there to keep our bodies in checks and balances. But when our bodies are immune compromised, or if our natural flora is disturbed or minimized, the organisms take over. Kind of like the books Animal Farm or Planet of the Apes lol. In any case , I thought it a pretty good overview.
Parasites.
Kritts