Intestinal parasites don't become airborn. If you have intestinal parasites, the doctor can test you for same. I assume he's done that and hasn't found any parasites. Look for other causes. Intestinal parasites would only be contagious if someone came in contact with your stool or you had anal sex. At least that's all I can find on Google. And, God knows, the INTERNET never lies.
Do some Googling around. If your doctor doesn't
believe you, it's because all your tests are negative. In that case, he needs to look for something else. Who diagnosed you with this? Go back to see that doctor.