I agree, its definitely possible to have parasites for years with nothing being done to get rid of them.
However I personally believe that parasites only cause major issues when the body's inner environment is out of balance; or in other words parasites are another symptoms of a larger problem.
It is really crazy to me though to think how much attention dogs/cats and other animals get towards parasites and humans get none... weird to say the least.
I good parasite cleanse should help you in either case.