I think it's just that the doctors are so ultra- skeptical that parasites happen more than they realize in the civilized World that they dont take the patient seriously.
Many doctors also have a low opinion of most of thier patients because they consider us all uneducated-- especially if you are a blue-collar working class person.
They feel that any education we in the "masses" pick up is dangerous in our hands so if you mention parasites, they'll say-- "you read too much".
They quietly think wer'e all hypochondriacs wasting their time. If a patient asserts themselves, their true feelings of contempt show.