as other's have said you need to make sure your body and immune system are in shape. the big question for me is what allows parasites to run wild in the first place.
do you have a good diet? little processed food. few white things like flour, sugar. get the macro nutrients in place first.
then look at common micro nutrition deficiencies like d3, k2, and iodine.
other unexpected areas:
dental health (root canals, Amalgam fillings, dental work that marks the beginning of troubles).
EMF are you constantly bombarded with electromagnetic fields? cell tower? airport? military radar? sometimes it's hard to tell.
water..i'm tempted to say distill your own or reverse osmosis it.