Thank you for visiting JustAnswer, and welcome.
You have a very interesting question. Since there is plenty of information about human parasitic infestations and problems within the United States, I would like to ask you how many physicians have told you there is no problem in the U.S.
The other thing I would advise is---if this physician is your physician, perhaps you should find another.
Anyway, could you further explain your problem? Is there something you need help with, specifically?
What kinds of testing have you had for your symptoms? What kinds of symptoms do you have? And--what kind of infection do you suspect you have?
If you are experiencing gastrointestinal symptoms, and believe you are expelling parasitic debris in your stools, it seems that it would be a simple enough test to have this properly diagnosed. You mentioned round worms before---is this what you are expelling?
For either of those types of infections the testing is done with a stool sample. Sometimes you may need a barium enema or other imaging studies of the bowel to look for any damage done. But the stool sample is tested first. There is also a "string test", in which you swallow a capsule with a special type of string in it. One end is attached to your cheek. The string is long enough to reach the small intestine, and after some time it is pulled out and examined microscopically for any debris from parasites or worms. Giardiasis is another very common parasitic infection that can cause chronic problems, and a blood test can be done for this.
Here is a genuine medical site that talks about these things: University of Maryland's page about Intestinal Parasites. It's too bad, most of what I could find for your reference were sites that are trying to sell something.
Here is a page from Mayo Clinic about Detection of Intestinal parasites. I have not watched this yet (but I'm going to), but Mayo Clinic has good information so there may be something here that is useful to you.
I really wish you the best in this. I hope you can get a confirmation one way or the other about your problems. If I find anymore useful information for you, I will pass it along.
Please know, however. There are areas within the United States that have terrible problems with worms and parasites, especially with children. I just think most people would rather not think about how easy it can be to contract an infection like this.