You have already been provided with an explanation as to why doctors do not recognize parasites.
BTW, Inflamatory bowel disease is mostly caused by microscopic parasites, especially candida ( a yeast) and also some protozoa as well as bacteria. Another thing that doctors often misconstrue from their education is that parasites means worms. It does not.
Also, there are 400 different parasites that will inhabit the human host. We may not be their favorite host, but many parasites are opportunists.